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add idl4k kernel firmware version 1.13.0.105

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Jaroslav Kysela
2015-03-26 17:22:37 +01:00
parent 5194d2792e
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00-INDEX
- This file
as-iosched.txt
- Anticipatory IO scheduler
barrier.txt
- I/O Barriers
biodoc.txt
- Notes on the Generic Block Layer Rewrite in Linux 2.5
capability.txt
- Generic Block Device Capability (/sys/block/<disk>/capability)
deadline-iosched.txt
- Deadline IO scheduler tunables
ioprio.txt
- Block io priorities (in CFQ scheduler)
request.txt
- The members of struct request (in include/linux/blkdev.h)
stat.txt
- Block layer statistics in /sys/block/<dev>/stat
switching-sched.txt
- Switching I/O schedulers at runtime

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Anticipatory IO scheduler
-------------------------
Nick Piggin <piggin@cyberone.com.au> 13 Sep 2003
Attention! Database servers, especially those using "TCQ" disks should
investigate performance with the 'deadline' IO scheduler. Any system with high
disk performance requirements should do so, in fact.
If you see unusual performance characteristics of your disk systems, or you
see big performance regressions versus the deadline scheduler, please email
me. Database users don't bother unless you're willing to test a lot of patches
from me ;) its a known issue.
Also, users with hardware RAID controllers, doing striping, may find
highly variable performance results with using the as-iosched. The
as-iosched anticipatory implementation is based on the notion that a disk
device has only one physical seeking head. A striped RAID controller
actually has a head for each physical device in the logical RAID device.
However, setting the antic_expire (see tunable parameters below) produces
very similar behavior to the deadline IO scheduler.
Selecting IO schedulers
-----------------------
Refer to Documentation/block/switching-sched.txt for information on
selecting an io scheduler on a per-device basis.
Anticipatory IO scheduler Policies
----------------------------------
The as-iosched implementation implements several layers of policies
to determine when an IO request is dispatched to the disk controller.
Here are the policies outlined, in order of application.
1. one-way Elevator algorithm.
The elevator algorithm is similar to that used in deadline scheduler, with
the addition that it allows limited backward movement of the elevator
(i.e. seeks backwards). A seek backwards can occur when choosing between
two IO requests where one is behind the elevator's current position, and
the other is in front of the elevator's position. If the seek distance to
the request in back of the elevator is less than half the seek distance to
the request in front of the elevator, then the request in back can be chosen.
Backward seeks are also limited to a maximum of MAXBACK (1024*1024) sectors.
This favors forward movement of the elevator, while allowing opportunistic
"short" backward seeks.
2. FIFO expiration times for reads and for writes.
This is again very similar to the deadline IO scheduler. The expiration
times for requests on these lists is tunable using the parameters read_expire
and write_expire discussed below. When a read or a write expires in this way,
the IO scheduler will interrupt its current elevator sweep or read anticipation
to service the expired request.
3. Read and write request batching
A batch is a collection of read requests or a collection of write
requests. The as scheduler alternates dispatching read and write batches
to the driver. In the case a read batch, the scheduler submits read
requests to the driver as long as there are read requests to submit, and
the read batch time limit has not been exceeded (read_batch_expire).
The read batch time limit begins counting down only when there are
competing write requests pending.
In the case of a write batch, the scheduler submits write requests to
the driver as long as there are write requests available, and the
write batch time limit has not been exceeded (write_batch_expire).
However, the length of write batches will be gradually shortened
when read batches frequently exceed their time limit.
When changing between batch types, the scheduler waits for all requests
from the previous batch to complete before scheduling requests for the
next batch.
The read and write fifo expiration times described in policy 2 above
are checked only when in scheduling IO of a batch for the corresponding
(read/write) type. So for example, the read FIFO timeout values are
tested only during read batches. Likewise, the write FIFO timeout
values are tested only during write batches. For this reason,
it is generally not recommended for the read batch time
to be longer than the write expiration time, nor for the write batch
time to exceed the read expiration time (see tunable parameters below).
When the IO scheduler changes from a read to a write batch,
it begins the elevator from the request that is on the head of the
write expiration FIFO. Likewise, when changing from a write batch to
a read batch, scheduler begins the elevator from the first entry
on the read expiration FIFO.
4. Read anticipation.
Read anticipation occurs only when scheduling a read batch.
This implementation of read anticipation allows only one read request
to be dispatched to the disk controller at a time. In
contrast, many write requests may be dispatched to the disk controller
at a time during a write batch. It is this characteristic that can make
the anticipatory scheduler perform anomalously with controllers supporting
TCQ, or with hardware striped RAID devices. Setting the antic_expire
queue parameter (see below) to zero disables this behavior, and the
anticipatory scheduler behaves essentially like the deadline scheduler.
When read anticipation is enabled (antic_expire is not zero), reads
are dispatched to the disk controller one at a time.
At the end of each read request, the IO scheduler examines its next
candidate read request from its sorted read list. If that next request
is from the same process as the request that just completed,
or if the next request in the queue is "very close" to the
just completed request, it is dispatched immediately. Otherwise,
statistics (average think time, average seek distance) on the process
that submitted the just completed request are examined. If it seems
likely that that process will submit another request soon, and that
request is likely to be near the just completed request, then the IO
scheduler will stop dispatching more read requests for up to (antic_expire)
milliseconds, hoping that process will submit a new request near the one
that just completed. If such a request is made, then it is dispatched
immediately. If the antic_expire wait time expires, then the IO scheduler
will dispatch the next read request from the sorted read queue.
To decide whether an anticipatory wait is worthwhile, the scheduler
maintains statistics for each process that can be used to compute
mean "think time" (the time between read requests), and mean seek
distance for that process. One observation is that these statistics
are associated with each process, but those statistics are not associated
with a specific IO device. So for example, if a process is doing IO
on several file systems on separate devices, the statistics will be
a combination of IO behavior from all those devices.
Tuning the anticipatory IO scheduler
------------------------------------
When using 'as', the anticipatory IO scheduler there are 5 parameters under
/sys/block/*/queue/iosched/. All are units of milliseconds.
The parameters are:
* read_expire
Controls how long until a read request becomes "expired". It also controls the
interval between which expired requests are served, so set to 50, a request
might take anywhere < 100ms to be serviced _if_ it is the next on the
expired list. Obviously request expiration strategies won't make the disk
go faster. The result basically equates to the timeslice a single reader
gets in the presence of other IO. 100*((seek time / read_expire) + 1) is
very roughly the % streaming read efficiency your disk should get with
multiple readers.
* read_batch_expire
Controls how much time a batch of reads is given before pending writes are
served. A higher value is more efficient. This might be set below read_expire
if writes are to be given higher priority than reads, but reads are to be
as efficient as possible when there are no writes. Generally though, it
should be some multiple of read_expire.
* write_expire, and
* write_batch_expire are equivalent to the above, for writes.
* antic_expire
Controls the maximum amount of time we can anticipate a good read (one
with a short seek distance from the most recently completed request) before
giving up. Many other factors may cause anticipation to be stopped early,
or some processes will not be "anticipated" at all. Should be a bit higher
for big seek time devices though not a linear correspondence - most
processes have only a few ms thinktime.
In addition to the tunables above there is a read-only file named est_time
which, when read, will show:
- The probability of a task exiting without a cooperating task
submitting an anticipated IO.
- The current mean think time.
- The seek distance used to determine if an incoming IO is better.

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I/O Barriers
============
Tejun Heo <htejun@gmail.com>, July 22 2005
I/O barrier requests are used to guarantee ordering around the barrier
requests. Unless you're crazy enough to use disk drives for
implementing synchronization constructs (wow, sounds interesting...),
the ordering is meaningful only for write requests for things like
journal checkpoints. All requests queued before a barrier request
must be finished (made it to the physical medium) before the barrier
request is started, and all requests queued after the barrier request
must be started only after the barrier request is finished (again,
made it to the physical medium).
In other words, I/O barrier requests have the following two properties.
1. Request ordering
Requests cannot pass the barrier request. Preceding requests are
processed before the barrier and following requests after.
Depending on what features a drive supports, this can be done in one
of the following three ways.
i. For devices which have queue depth greater than 1 (TCQ devices) and
support ordered tags, block layer can just issue the barrier as an
ordered request and the lower level driver, controller and drive
itself are responsible for making sure that the ordering constraint is
met. Most modern SCSI controllers/drives should support this.
NOTE: SCSI ordered tag isn't currently used due to limitation in the
SCSI midlayer, see the following random notes section.
ii. For devices which have queue depth greater than 1 but don't
support ordered tags, block layer ensures that the requests preceding
a barrier request finishes before issuing the barrier request. Also,
it defers requests following the barrier until the barrier request is
finished. Older SCSI controllers/drives and SATA drives fall in this
category.
iii. Devices which have queue depth of 1. This is a degenerate case
of ii. Just keeping issue order suffices. Ancient SCSI
controllers/drives and IDE drives are in this category.
2. Forced flushing to physical medium
Again, if you're not gonna do synchronization with disk drives (dang,
it sounds even more appealing now!), the reason you use I/O barriers
is mainly to protect filesystem integrity when power failure or some
other events abruptly stop the drive from operating and possibly make
the drive lose data in its cache. So, I/O barriers need to guarantee
that requests actually get written to non-volatile medium in order.
There are four cases,
i. No write-back cache. Keeping requests ordered is enough.
ii. Write-back cache but no flush operation. There's no way to
guarantee physical-medium commit order. This kind of devices can't to
I/O barriers.
iii. Write-back cache and flush operation but no FUA (forced unit
access). We need two cache flushes - before and after the barrier
request.
iv. Write-back cache, flush operation and FUA. We still need one
flush to make sure requests preceding a barrier are written to medium,
but post-barrier flush can be avoided by using FUA write on the
barrier itself.
How to support barrier requests in drivers
------------------------------------------
All barrier handling is done inside block layer proper. All low level
drivers have to are implementing its prepare_flush_fn and using one
the following two functions to indicate what barrier type it supports
and how to prepare flush requests. Note that the term 'ordered' is
used to indicate the whole sequence of performing barrier requests
including draining and flushing.
typedef void (prepare_flush_fn)(struct request_queue *q, struct request *rq);
int blk_queue_ordered(struct request_queue *q, unsigned ordered,
prepare_flush_fn *prepare_flush_fn);
@q : the queue in question
@ordered : the ordered mode the driver/device supports
@prepare_flush_fn : this function should prepare @rq such that it
flushes cache to physical medium when executed
For example, SCSI disk driver's prepare_flush_fn looks like the
following.
static void sd_prepare_flush(struct request_queue *q, struct request *rq)
{
memset(rq->cmd, 0, sizeof(rq->cmd));
rq->cmd_type = REQ_TYPE_BLOCK_PC;
rq->timeout = SD_TIMEOUT;
rq->cmd[0] = SYNCHRONIZE_CACHE;
rq->cmd_len = 10;
}
The following seven ordered modes are supported. The following table
shows which mode should be used depending on what features a
device/driver supports. In the leftmost column of table,
QUEUE_ORDERED_ prefix is omitted from the mode names to save space.
The table is followed by description of each mode. Note that in the
descriptions of QUEUE_ORDERED_DRAIN*, '=>' is used whereas '->' is
used for QUEUE_ORDERED_TAG* descriptions. '=>' indicates that the
preceding step must be complete before proceeding to the next step.
'->' indicates that the next step can start as soon as the previous
step is issued.
write-back cache ordered tag flush FUA
-----------------------------------------------------------------------
NONE yes/no N/A no N/A
DRAIN no no N/A N/A
DRAIN_FLUSH yes no yes no
DRAIN_FUA yes no yes yes
TAG no yes N/A N/A
TAG_FLUSH yes yes yes no
TAG_FUA yes yes yes yes
QUEUE_ORDERED_NONE
I/O barriers are not needed and/or supported.
Sequence: N/A
QUEUE_ORDERED_DRAIN
Requests are ordered by draining the request queue and cache
flushing isn't needed.
Sequence: drain => barrier
QUEUE_ORDERED_DRAIN_FLUSH
Requests are ordered by draining the request queue and both
pre-barrier and post-barrier cache flushings are needed.
Sequence: drain => preflush => barrier => postflush
QUEUE_ORDERED_DRAIN_FUA
Requests are ordered by draining the request queue and
pre-barrier cache flushing is needed. By using FUA on barrier
request, post-barrier flushing can be skipped.
Sequence: drain => preflush => barrier
QUEUE_ORDERED_TAG
Requests are ordered by ordered tag and cache flushing isn't
needed.
Sequence: barrier
QUEUE_ORDERED_TAG_FLUSH
Requests are ordered by ordered tag and both pre-barrier and
post-barrier cache flushings are needed.
Sequence: preflush -> barrier -> postflush
QUEUE_ORDERED_TAG_FUA
Requests are ordered by ordered tag and pre-barrier cache
flushing is needed. By using FUA on barrier request,
post-barrier flushing can be skipped.
Sequence: preflush -> barrier
Random notes/caveats
--------------------
* SCSI layer currently can't use TAG ordering even if the drive,
controller and driver support it. The problem is that SCSI midlayer
request dispatch function is not atomic. It releases queue lock and
switch to SCSI host lock during issue and it's possible and likely to
happen in time that requests change their relative positions. Once
this problem is solved, TAG ordering can be enabled.
* Currently, no matter which ordered mode is used, there can be only
one barrier request in progress. All I/O barriers are held off by
block layer until the previous I/O barrier is complete. This doesn't
make any difference for DRAIN ordered devices, but, for TAG ordered
devices with very high command latency, passing multiple I/O barriers
to low level *might* be helpful if they are very frequent. Well, this
certainly is a non-issue. I'm writing this just to make clear that no
two I/O barrier is ever passed to low-level driver.
* Completion order. Requests in ordered sequence are issued in order
but not required to finish in order. Barrier implementation can
handle out-of-order completion of ordered sequence. IOW, the requests
MUST be processed in order but the hardware/software completion paths
are allowed to reorder completion notifications - eg. current SCSI
midlayer doesn't preserve completion order during error handling.
* Requeueing order. Low-level drivers are free to requeue any request
after they removed it from the request queue with
blkdev_dequeue_request(). As barrier sequence should be kept in order
when requeued, generic elevator code takes care of putting requests in
order around barrier. See blk_ordered_req_seq() and
ELEVATOR_INSERT_REQUEUE handling in __elv_add_request() for details.
Note that block drivers must not requeue preceding requests while
completing latter requests in an ordered sequence. Currently, no
error checking is done against this.
* Error handling. Currently, block layer will report error to upper
layer if any of requests in an ordered sequence fails. Unfortunately,
this doesn't seem to be enough. Look at the following request flow.
QUEUE_ORDERED_TAG_FLUSH is in use.
[0] [1] [2] [3] [pre] [barrier] [post] < [4] [5] [6] ... >
still in elevator
Let's say request [2], [3] are write requests to update file system
metadata (journal or whatever) and [barrier] is used to mark that
those updates are valid. Consider the following sequence.
i. Requests [0] ~ [post] leaves the request queue and enters
low-level driver.
ii. After a while, unfortunately, something goes wrong and the
drive fails [2]. Note that any of [0], [1] and [3] could have
completed by this time, but [pre] couldn't have been finished
as the drive must process it in order and it failed before
processing that command.
iii. Error handling kicks in and determines that the error is
unrecoverable and fails [2], and resumes operation.
iv. [pre] [barrier] [post] gets processed.
v. *BOOM* power fails
The problem here is that the barrier request is *supposed* to indicate
that filesystem update requests [2] and [3] made it safely to the
physical medium and, if the machine crashes after the barrier is
written, filesystem recovery code can depend on that. Sadly, that
isn't true in this case anymore. IOW, the success of a I/O barrier
should also be dependent on success of some of the preceding requests,
where only upper layer (filesystem) knows what 'some' is.
This can be solved by implementing a way to tell the block layer which
requests affect the success of the following barrier request and
making lower lever drivers to resume operation on error only after
block layer tells it to do so.
As the probability of this happening is very low and the drive should
be faulty, implementing the fix is probably an overkill. But, still,
it's there.
* In previous drafts of barrier implementation, there was fallback
mechanism such that, if FUA or ordered TAG fails, less fancy ordered
mode can be selected and the failed barrier request is retried
automatically. The rationale for this feature was that as FUA is
pretty new in ATA world and ordered tag was never used widely, there
could be devices which report to support those features but choke when
actually given such requests.
This was removed for two reasons 1. it's an overkill 2. it's
impossible to implement properly when TAG ordering is used as low
level drivers resume after an error automatically. If it's ever
needed adding it back and modifying low level drivers accordingly
shouldn't be difficult.

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Generic Block Device Capability
===============================================================================
This file documents the sysfs file block/<disk>/capability
capability is a hex word indicating which capabilities a specific disk
supports. For more information on bits not listed here, see
include/linux/genhd.h
Capability Value
-------------------------------------------------------------------------------
GENHD_FL_MEDIA_CHANGE_NOTIFY 4
When this bit is set, the disk supports Asynchronous Notification
of media change events. These events will be broadcast to user
space via kernel uevent.

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----------------------------------------------------------------------
1. INTRODUCTION
Modern filesystems feature checksumming of data and metadata to
protect against data corruption. However, the detection of the
corruption is done at read time which could potentially be months
after the data was written. At that point the original data that the
application tried to write is most likely lost.
The solution is to ensure that the disk is actually storing what the
application meant it to. Recent additions to both the SCSI family
protocols (SBC Data Integrity Field, SCC protection proposal) as well
as SATA/T13 (External Path Protection) try to remedy this by adding
support for appending integrity metadata to an I/O. The integrity
metadata (or protection information in SCSI terminology) includes a
checksum for each sector as well as an incrementing counter that
ensures the individual sectors are written in the right order. And
for some protection schemes also that the I/O is written to the right
place on disk.
Current storage controllers and devices implement various protective
measures, for instance checksumming and scrubbing. But these
technologies are working in their own isolated domains or at best
between adjacent nodes in the I/O path. The interesting thing about
DIF and the other integrity extensions is that the protection format
is well defined and every node in the I/O path can verify the
integrity of the I/O and reject it if corruption is detected. This
allows not only corruption prevention but also isolation of the point
of failure.
----------------------------------------------------------------------
2. THE DATA INTEGRITY EXTENSIONS
As written, the protocol extensions only protect the path between
controller and storage device. However, many controllers actually
allow the operating system to interact with the integrity metadata
(IMD). We have been working with several FC/SAS HBA vendors to enable
the protection information to be transferred to and from their
controllers.
The SCSI Data Integrity Field works by appending 8 bytes of protection
information to each sector. The data + integrity metadata is stored
in 520 byte sectors on disk. Data + IMD are interleaved when
transferred between the controller and target. The T13 proposal is
similar.
Because it is highly inconvenient for operating systems to deal with
520 (and 4104) byte sectors, we approached several HBA vendors and
encouraged them to allow separation of the data and integrity metadata
scatter-gather lists.
The controller will interleave the buffers on write and split them on
read. This means that Linux can DMA the data buffers to and from
host memory without changes to the page cache.
Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
is somewhat heavy to compute in software. Benchmarks found that
calculating this checksum had a significant impact on system
performance for a number of workloads. Some controllers allow a
lighter-weight checksum to be used when interfacing with the operating
system. Emulex, for instance, supports the TCP/IP checksum instead.
The IP checksum received from the OS is converted to the 16-bit CRC
when writing and vice versa. This allows the integrity metadata to be
generated by Linux or the application at very low cost (comparable to
software RAID5).
The IP checksum is weaker than the CRC in terms of detecting bit
errors. However, the strength is really in the separation of the data
buffers and the integrity metadata. These two distinct buffers must
match up for an I/O to complete.
The separation of the data and integrity metadata buffers as well as
the choice in checksums is referred to as the Data Integrity
Extensions. As these extensions are outside the scope of the protocol
bodies (T10, T13), Oracle and its partners are trying to standardize
them within the Storage Networking Industry Association.
----------------------------------------------------------------------
3. KERNEL CHANGES
The data integrity framework in Linux enables protection information
to be pinned to I/Os and sent to/received from controllers that
support it.
The advantage to the integrity extensions in SCSI and SATA is that
they enable us to protect the entire path from application to storage
device. However, at the same time this is also the biggest
disadvantage. It means that the protection information must be in a
format that can be understood by the disk.
Generally Linux/POSIX applications are agnostic to the intricacies of
the storage devices they are accessing. The virtual filesystem switch
and the block layer make things like hardware sector size and
transport protocols completely transparent to the application.
However, this level of detail is required when preparing the
protection information to send to a disk. Consequently, the very
concept of an end-to-end protection scheme is a layering violation.
It is completely unreasonable for an application to be aware whether
it is accessing a SCSI or SATA disk.
The data integrity support implemented in Linux attempts to hide this
from the application. As far as the application (and to some extent
the kernel) is concerned, the integrity metadata is opaque information
that's attached to the I/O.
The current implementation allows the block layer to automatically
generate the protection information for any I/O. Eventually the
intent is to move the integrity metadata calculation to userspace for
user data. Metadata and other I/O that originates within the kernel
will still use the automatic generation interface.
Some storage devices allow each hardware sector to be tagged with a
16-bit value. The owner of this tag space is the owner of the block
device. I.e. the filesystem in most cases. The filesystem can use
this extra space to tag sectors as they see fit. Because the tag
space is limited, the block interface allows tagging bigger chunks by
way of interleaving. This way, 8*16 bits of information can be
attached to a typical 4KB filesystem block.
This also means that applications such as fsck and mkfs will need
access to manipulate the tags from user space. A passthrough
interface for this is being worked on.
----------------------------------------------------------------------
4. BLOCK LAYER IMPLEMENTATION DETAILS
4.1 BIO
The data integrity patches add a new field to struct bio when
CONFIG_BLK_DEV_INTEGRITY is enabled. bio->bi_integrity is a pointer
to a struct bip which contains the bio integrity payload. Essentially
a bip is a trimmed down struct bio which holds a bio_vec containing
the integrity metadata and the required housekeeping information (bvec
pool, vector count, etc.)
A kernel subsystem can enable data integrity protection on a bio by
calling bio_integrity_alloc(bio). This will allocate and attach the
bip to the bio.
Individual pages containing integrity metadata can subsequently be
attached using bio_integrity_add_page().
bio_free() will automatically free the bip.
4.2 BLOCK DEVICE
Because the format of the protection data is tied to the physical
disk, each block device has been extended with a block integrity
profile (struct blk_integrity). This optional profile is registered
with the block layer using blk_integrity_register().
The profile contains callback functions for generating and verifying
the protection data, as well as getting and setting application tags.
The profile also contains a few constants to aid in completing,
merging and splitting the integrity metadata.
Layered block devices will need to pick a profile that's appropriate
for all subdevices. blk_integrity_compare() can help with that. DM
and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
will require extra work due to the application tag.
----------------------------------------------------------------------
5.0 BLOCK LAYER INTEGRITY API
5.1 NORMAL FILESYSTEM
The normal filesystem is unaware that the underlying block device
is capable of sending/receiving integrity metadata. The IMD will
be automatically generated by the block layer at submit_bio() time
in case of a WRITE. A READ request will cause the I/O integrity
to be verified upon completion.
IMD generation and verification can be toggled using the
/sys/block/<bdev>/integrity/write_generate
and
/sys/block/<bdev>/integrity/read_verify
flags.
5.2 INTEGRITY-AWARE FILESYSTEM
A filesystem that is integrity-aware can prepare I/Os with IMD
attached. It can also use the application tag space if this is
supported by the block device.
int bdev_integrity_enabled(block_device, int rw);
bdev_integrity_enabled() will return 1 if the block device
supports integrity metadata transfer for the data direction
specified in 'rw'.
bdev_integrity_enabled() honors the write_generate and
read_verify flags in sysfs and will respond accordingly.
int bio_integrity_prep(bio);
To generate IMD for WRITE and to set up buffers for READ, the
filesystem must call bio_integrity_prep(bio).
Prior to calling this function, the bio data direction and start
sector must be set, and the bio should have all data pages
added. It is up to the caller to ensure that the bio does not
change while I/O is in progress.
bio_integrity_prep() should only be called if
bio_integrity_enabled() returned 1.
int bio_integrity_tag_size(bio);
If the filesystem wants to use the application tag space it will
first have to find out how much storage space is available.
Because tag space is generally limited (usually 2 bytes per
sector regardless of sector size), the integrity framework
supports interleaving the information between the sectors in an
I/O.
Filesystems can call bio_integrity_tag_size(bio) to find out how
many bytes of storage are available for that particular bio.
Another option is bdev_get_tag_size(block_device) which will
return the number of available bytes per hardware sector.
int bio_integrity_set_tag(bio, void *tag_buf, len);
After a successful return from bio_integrity_prep(),
bio_integrity_set_tag() can be used to attach an opaque tag
buffer to a bio. Obviously this only makes sense if the I/O is
a WRITE.
int bio_integrity_get_tag(bio, void *tag_buf, len);
Similarly, at READ I/O completion time the filesystem can
retrieve the tag buffer using bio_integrity_get_tag().
5.3 PASSING EXISTING INTEGRITY METADATA
Filesystems that either generate their own integrity metadata or
are capable of transferring IMD from user space can use the
following calls:
struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
Allocates the bio integrity payload and hangs it off of the bio.
nr_pages indicate how many pages of protection data need to be
stored in the integrity bio_vec list (similar to bio_alloc()).
The integrity payload will be freed at bio_free() time.
int bio_integrity_add_page(bio, page, len, offset);
Attaches a page containing integrity metadata to an existing
bio. The bio must have an existing bip,
i.e. bio_integrity_alloc() must have been called. For a WRITE,
the integrity metadata in the pages must be in a format
understood by the target device with the notable exception that
the sector numbers will be remapped as the request traverses the
I/O stack. This implies that the pages added using this call
will be modified during I/O! The first reference tag in the
integrity metadata must have a value of bip->bip_sector.
Pages can be added using bio_integrity_add_page() as long as
there is room in the bip bio_vec array (nr_pages).
Upon completion of a READ operation, the attached pages will
contain the integrity metadata received from the storage device.
It is up to the receiver to process them and verify data
integrity upon completion.
5.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
METADATA
To enable integrity exchange on a block device the gendisk must be
registered as capable:
int blk_integrity_register(gendisk, blk_integrity);
The blk_integrity struct is a template and should contain the
following:
static struct blk_integrity my_profile = {
.name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
.generate_fn = my_generate_fn,
.verify_fn = my_verify_fn,
.get_tag_fn = my_get_tag_fn,
.set_tag_fn = my_set_tag_fn,
.tuple_size = sizeof(struct my_tuple_size),
.tag_size = <tag bytes per hw sector>,
};
'name' is a text string which will be visible in sysfs. This is
part of the userland API so chose it carefully and never change
it. The format is standards body-type-variant.
E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
'generate_fn' generates appropriate integrity metadata (for WRITE).
'verify_fn' verifies that the data buffer matches the integrity
metadata.
'tuple_size' must be set to match the size of the integrity
metadata per sector. I.e. 8 for DIF and EPP.
'tag_size' must be set to identify how many bytes of tag space
are available per hardware sector. For DIF this is either 2 or
0 depending on the value of the Control Mode Page ATO bit.
See 6.2 for a description of get_tag_fn and set_tag_fn.
----------------------------------------------------------------------
2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>

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Deadline IO scheduler tunables
==============================
This little file attempts to document how the deadline io scheduler works.
In particular, it will clarify the meaning of the exposed tunables that may be
of interest to power users.
Selecting IO schedulers
-----------------------
Refer to Documentation/block/switching-sched.txt for information on
selecting an io scheduler on a per-device basis.
********************************************************************************
read_expire (in ms)
-----------
The goal of the deadline io scheduler is to attempt to guarantee a start
service time for a request. As we focus mainly on read latencies, this is
tunable. When a read request first enters the io scheduler, it is assigned
a deadline that is the current time + the read_expire value in units of
milliseconds.
write_expire (in ms)
-----------
Similar to read_expire mentioned above, but for writes.
fifo_batch (number of requests)
----------
Requests are grouped into ``batches'' of a particular data direction (read or
write) which are serviced in increasing sector order. To limit extra seeking,
deadline expiries are only checked between batches. fifo_batch controls the
maximum number of requests per batch.
This parameter tunes the balance between per-request latency and aggregate
throughput. When low latency is the primary concern, smaller is better (where
a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
generally improves throughput, at the cost of latency variation.
writes_starved (number of dispatches)
--------------
When we have to move requests from the io scheduler queue to the block
device dispatch queue, we always give a preference to reads. However, we
don't want to starve writes indefinitely either. So writes_starved controls
how many times we give preference to reads over writes. When that has been
done writes_starved number of times, we dispatch some writes based on the
same criteria as reads.
front_merges (bool)
------------
Sometimes it happens that a request enters the io scheduler that is contiguous
with a request that is already on the queue. Either it fits in the back of that
request, or it fits at the front. That is called either a back merge candidate
or a front merge candidate. Due to the way files are typically laid out,
back merges are much more common than front merges. For some work loads, you
may even know that it is a waste of time to spend any time attempting to
front merge requests. Setting front_merges to 0 disables this functionality.
Front merges may still occur due to the cached last_merge hint, but since
that comes at basically 0 cost we leave that on. We simply disable the
rbtree front sector lookup when the io scheduler merge function is called.
Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>

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Block io priorities
===================
Intro
-----
With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
priorities are supported for reads on files. This enables users to io nice
processes or process groups, similar to what has been possible with cpu
scheduling for ages. This document mainly details the current possibilities
with cfq; other io schedulers do not support io priorities thus far.
Scheduling classes
------------------
CFQ implements three generic scheduling classes that determine how io is
served for a process.
IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given
higher priority than any other in the system, processes from this class are
given first access to the disk every time. Thus it needs to be used with some
care, one io RT process can starve the entire system. Within the RT class,
there are 8 levels of class data that determine exactly how much time this
process needs the disk for on each service. In the future this might change
to be more directly mappable to performance, by passing in a wanted data
rate instead.
IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default
for any process that hasn't set a specific io priority. The class data
determines how much io bandwidth the process will get, it's directly mappable
to the cpu nice levels just more coarsely implemented. 0 is the highest
BE prio level, 7 is the lowest. The mapping between cpu nice level and io
nice level is determined as: io_nice = (cpu_nice + 20) / 5.
IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this
level only get io time when no one else needs the disk. The idle class has no
class data, since it doesn't really apply here.
Tools
-----
See below for a sample ionice tool. Usage:
# ionice -c<class> -n<level> -p<pid>
If pid isn't given, the current process is assumed. IO priority settings
are inherited on fork, so you can use ionice to start the process at a given
level:
# ionice -c2 -n0 /bin/ls
will run ls at the best-effort scheduling class at the highest priority.
For a running process, you can give the pid instead:
# ionice -c1 -n2 -p100
will change pid 100 to run at the realtime scheduling class, at priority 2.
---> snip ionice.c tool <---
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <getopt.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <asm/unistd.h>
extern int sys_ioprio_set(int, int, int);
extern int sys_ioprio_get(int, int);
#if defined(__i386__)
#define __NR_ioprio_set 289
#define __NR_ioprio_get 290
#elif defined(__ppc__)
#define __NR_ioprio_set 273
#define __NR_ioprio_get 274
#elif defined(__x86_64__)
#define __NR_ioprio_set 251
#define __NR_ioprio_get 252
#elif defined(__ia64__)
#define __NR_ioprio_set 1274
#define __NR_ioprio_get 1275
#else
#error "Unsupported arch"
#endif
static inline int ioprio_set(int which, int who, int ioprio)
{
return syscall(__NR_ioprio_set, which, who, ioprio);
}
static inline int ioprio_get(int which, int who)
{
return syscall(__NR_ioprio_get, which, who);
}
enum {
IOPRIO_CLASS_NONE,
IOPRIO_CLASS_RT,
IOPRIO_CLASS_BE,
IOPRIO_CLASS_IDLE,
};
enum {
IOPRIO_WHO_PROCESS = 1,
IOPRIO_WHO_PGRP,
IOPRIO_WHO_USER,
};
#define IOPRIO_CLASS_SHIFT 13
const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
int main(int argc, char *argv[])
{
int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
int c, pid = 0;
while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) {
switch (c) {
case 'n':
ioprio = strtol(optarg, NULL, 10);
set = 1;
break;
case 'c':
ioprio_class = strtol(optarg, NULL, 10);
set = 1;
break;
case 'p':
pid = strtol(optarg, NULL, 10);
break;
}
}
switch (ioprio_class) {
case IOPRIO_CLASS_NONE:
ioprio_class = IOPRIO_CLASS_BE;
break;
case IOPRIO_CLASS_RT:
case IOPRIO_CLASS_BE:
break;
case IOPRIO_CLASS_IDLE:
ioprio = 7;
break;
default:
printf("bad prio class %d\n", ioprio_class);
return 1;
}
if (!set) {
if (!pid && argv[optind])
pid = strtol(argv[optind], NULL, 10);
ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
printf("pid=%d, %d\n", pid, ioprio);
if (ioprio == -1)
perror("ioprio_get");
else {
ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT;
ioprio = ioprio & 0xff;
printf("%s: prio %d\n", to_prio[ioprio_class], ioprio);
}
} else {
if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) {
perror("ioprio_set");
return 1;
}
if (argv[optind])
execvp(argv[optind], &argv[optind]);
}
return 0;
}
---> snip ionice.c tool <---
March 11 2005, Jens Axboe <jens.axboe@oracle.com>

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Queue sysfs files
=================
This text file will detail the queue files that are located in the sysfs tree
for each block device. Note that stacked devices typically do not export
any settings, since their queue merely functions are a remapping target.
These files are the ones found in the /sys/block/xxx/queue/ directory.
Files denoted with a RO postfix are readonly and the RW postfix means
read-write.
hw_sector_size (RO)
-------------------
This is the hardware sector size of the device, in bytes.
max_hw_sectors_kb (RO)
----------------------
This is the maximum number of kilobytes supported in a single data transfer.
max_sectors_kb (RW)
-------------------
This is the maximum number of kilobytes that the block layer will allow
for a filesystem request. Must be smaller than or equal to the maximum
size allowed by the hardware.
nomerges (RW)
-------------
This enables the user to disable the lookup logic involved with IO merging
requests in the block layer. Merging may still occur through a direct
1-hit cache, since that comes for (almost) free. The IO scheduler will not
waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults
to 0, enabling all merges.
nr_requests (RW)
----------------
This controls how many requests may be allocated in the block layer for
read or write requests. Note that the total allocated number may be twice
this amount, since it applies only to reads or writes (not the accumulated
sum).
read_ahead_kb (RW)
------------------
Maximum number of kilobytes to read-ahead for filesystems on this block
device.
rq_affinity (RW)
----------------
If this option is enabled, the block layer will migrate request completions
to the CPU that originally submitted the request. For some workloads
this provides a significant reduction in CPU cycles due to caching effects.
scheduler (RW)
--------------
When read, this file will display the current and available IO schedulers
for this block device. The currently active IO scheduler will be enclosed
in [] brackets. Writing an IO scheduler name to this file will switch
control of this block device to that new IO scheduler. Note that writing
an IO scheduler name to this file will attempt to load that IO scheduler
module, if it isn't already present in the system.
Jens Axboe <jens.axboe@oracle.com>, February 2009

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struct request documentation
Jens Axboe <jens.axboe@oracle.com> 27/05/02
1.0
Index
2.0 Struct request members classification
2.1 struct request members explanation
3.0
2.0
Short explanation of request members
Classification flags:
D driver member
B block layer member
I I/O scheduler member
Unless an entry contains a D classification, a device driver must not access
this member. Some members may contain D classifications, but should only be
access through certain macros or functions (eg ->flags).
<linux/blkdev.h>
2.1
Member Flag Comment
------ ---- -------
struct list_head queuelist BI Organization on various internal
queues
void *elevator_private I I/O scheduler private data
unsigned char cmd[16] D Driver can use this for setting up
a cdb before execution, see
blk_queue_prep_rq
unsigned long flags DBI Contains info about data direction,
request type, etc.
int rq_status D Request status bits
kdev_t rq_dev DBI Target device
int errors DB Error counts
sector_t sector DBI Target location
unsigned long hard_nr_sectors B Used to keep sector sane
unsigned long nr_sectors DBI Total number of sectors in request
unsigned long hard_nr_sectors B Used to keep nr_sectors sane
unsigned short nr_phys_segments DB Number of physical scatter gather
segments in a request
unsigned short nr_hw_segments DB Number of hardware scatter gather
segments in a request
unsigned int current_nr_sectors DB Number of sectors in first segment
of request
unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane
int tag DB TCQ tag, if assigned
void *special D Free to be used by driver
char *buffer D Map of first segment, also see
section on bouncing SECTION
struct completion *waiting D Can be used by driver to get signalled
on request completion
struct bio *bio DBI First bio in request
struct bio *biotail DBI Last bio in request
struct request_queue *q DB Request queue this request belongs to
struct request_list *rl B Request list this request came from

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Block layer statistics in /sys/block/<dev>/stat
===============================================
This file documents the contents of the /sys/block/<dev>/stat file.
The stat file provides several statistics about the state of block
device <dev>.
Q. Why are there multiple statistics in a single file? Doesn't sysfs
normally contain a single value per file?
A. By having a single file, the kernel can guarantee that the statistics
represent a consistent snapshot of the state of the device. If the
statistics were exported as multiple files containing one statistic
each, it would be impossible to guarantee that a set of readings
represent a single point in time.
The stat file consists of a single line of text containing 11 decimal
values separated by whitespace. The fields are summarized in the
following table, and described in more detail below.
Name units description
---- ----- -----------
read I/Os requests number of read I/Os processed
read merges requests number of read I/Os merged with in-queue I/O
read sectors sectors number of sectors read
read ticks milliseconds total wait time for read requests
write I/Os requests number of write I/Os processed
write merges requests number of write I/Os merged with in-queue I/O
write sectors sectors number of sectors written
write ticks milliseconds total wait time for write requests
in_flight requests number of I/Os currently in flight
io_ticks milliseconds total time this block device has been active
time_in_queue milliseconds total wait time for all requests
read I/Os, write I/Os
=====================
These values increment when an I/O request completes.
read merges, write merges
=========================
These values increment when an I/O request is merged with an
already-queued I/O request.
read sectors, write sectors
===========================
These values count the number of sectors read from or written to this
block device. The "sectors" in question are the standard UNIX 512-byte
sectors, not any device- or filesystem-specific block size. The
counters are incremented when the I/O completes.
read ticks, write ticks
=======================
These values count the number of milliseconds that I/O requests have
waited on this block device. If there are multiple I/O requests waiting,
these values will increase at a rate greater than 1000/second; for
example, if 60 read requests wait for an average of 30 ms, the read_ticks
field will increase by 60*30 = 1800.
in_flight
=========
This value counts the number of I/O requests that have been issued to
the device driver but have not yet completed. It does not include I/O
requests that are in the queue but not yet issued to the device driver.
io_ticks
========
This value counts the number of milliseconds during which the device has
had I/O requests queued.
time_in_queue
=============
This value counts the number of milliseconds that I/O requests have waited
on this block device. If there are multiple I/O requests waiting, this
value will increase as the product of the number of milliseconds times the
number of requests waiting (see "read ticks" above for an example).

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To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
'noop', 'as' and 'cfq' (the default) are also available. IO schedulers are
assigned globally at boot time only presently.
Each io queue has a set of io scheduler tunables associated with it. These
tunables control how the io scheduler works. You can find these entries
in:
/sys/block/<device>/queue/iosched
assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
you can do so by typing:
# mount none /sys -t sysfs
As of the Linux 2.6.10 kernel, it is now possible to change the
IO scheduler for a given block device on the fly (thus making it possible,
for instance, to set the CFQ scheduler for the system default, but
set a specific device to use the anticipatory or noop schedulers - which
can improve that device's throughput).
To set a specific scheduler, simply do this:
echo SCHEDNAME > /sys/block/DEV/queue/scheduler
where SCHEDNAME is the name of a defined IO scheduler, and DEV is the
device name (hda, hdb, sga, or whatever you happen to have).
The list of defined schedulers can be found by simply doing
a "cat /sys/block/DEV/queue/scheduler" - the list of valid names
will be displayed, with the currently selected scheduler in brackets:
# cat /sys/block/hda/queue/scheduler
noop anticipatory deadline [cfq]
# echo anticipatory > /sys/block/hda/queue/scheduler
# cat /sys/block/hda/queue/scheduler
noop [anticipatory] deadline cfq