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>

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index 4873c75..c1eb41c 100644 (file)
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -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.