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block: Add sysfs documentation for the discard topology parameters 

Add documentation for the discard I/O topology parameters exported in
sysfs.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Martin K. Petersen 2011-05-18 10:37:39 +02:00 committed by Jens Axboe
parent a934a00a69
commit d70d0711ed
1 changed files with 64 additions and 0 deletions
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Documentation/ABI/testing/sysfs-block
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@ -142,3 +142,67 @@ Description:
with the previous I/O request are enabled. When set to 2,
all merge tries are disabled. The default value is 0 -
which enables all types of merge tries.
What: /sys/block/<disk>/discard_alignment
Date: May 2011
Contact: Martin K. Petersen <martin.petersen@oracle.com>
Description:
Devices that support discard functionality may
internally allocate space in units that are bigger than
the exported logical block size. The discard_alignment
parameter indicates how many bytes the beginning of the
device is offset from the internal allocation unit's
natural alignment.
What: /sys/block/<disk>/<partition>/discard_alignment
Date: May 2011
Contact: Martin K. Petersen <martin.petersen@oracle.com>
Description:
Devices that support discard functionality may
internally allocate space in units that are bigger than
the exported logical block size. The discard_alignment
parameter indicates how many bytes the beginning of the
partition is offset from the internal allocation unit's
natural alignment.
What: /sys/block/<disk>/queue/discard_granularity
Date: May 2011
Contact: Martin K. Petersen <martin.petersen@oracle.com>
Description:
Devices that support discard functionality may
internally allocate space using units that are bigger
than the logical block size. The discard_granularity
parameter indicates the size of the internal allocation
unit in bytes if reported by the device. Otherwise the
discard_granularity will be set to match the device's
physical block size. A discard_granularity of 0 means
that the device does not support discard functionality.
What: /sys/block/<disk>/queue/discard_max_bytes
Date: May 2011
Contact: Martin K. Petersen <martin.petersen@oracle.com>
Description:
Devices that support discard functionality may have
internal limits on the number of bytes that can be
trimmed or unmapped in a single operation. Some storage
protocols also have inherent limits on the number of
blocks that can be described in a single command. The
discard_max_bytes parameter is set by the device driver
to the maximum number of bytes that can be discarded in
a single operation. Discard requests issued to the
device must not exceed this limit. A discard_max_bytes
value of 0 means that the device does not support
discard functionality.
What: /sys/block/<disk>/queue/discard_zeroes_data
Date: May 2011
Contact: Martin K. Petersen <martin.petersen@oracle.com>
Description:
Devices that support discard functionality may return
stale or random data when a previously discarded block
is read back. This can cause problems if the filesystem
expects discarded blocks to be explicitly cleared. If a
device reports that it deterministically returns zeroes
when a discarded area is read the discard_zeroes_data
parameter will be set to one. Otherwise it will be 0 and
the result of reading a discarded area is undefined.