block: Add sysfs documentation for the discard topology parameters

		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.