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block: make /sys/block/<dev>/queue/discard_max_bytes writeable

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Lots of devices support huge discard sizes these days. Depending
on how the device handles them internally, huge discards can
introduce massive latencies (hundreds of msec) on the device side.

We have a sysfs file, discard_max_bytes, that advertises the max
hardware supported discard size. Make this writeable, and split
the settings into a soft and hard limit. This can be set from
'discard_granularity' and up to the hardware limit.

Add a new sysfs file, 'discard_max_hw_bytes', that shows the hw
set limit.

Reviewed-by: Jeff Moyer <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Jens Axboe 2015-07-16 09:14:26 -06:00
commit 0034af0365
4 changed files with 53 additions and 2 deletions
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10
Documentation/block/queue-sysfs.txt
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@ -20,7 +20,7 @@ This shows the size of internal allocation of the device in bytes, if
reported by the device. A value of '0' means device does not support
the discard functionality.
discard_max_bytes (RO)
discard_max_hw_bytes (RO)
----------------------
Devices that support discard functionality may have internal limits on
the number of bytes that can be trimmed or unmapped in a single operation.
@ -29,6 +29,14 @@ 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.
discard_max_bytes (RW)
----------------------
While discard_max_hw_bytes is the hardware limit for the device, this
setting is the software limit. Some devices exhibit large latencies when
large discards are issued, setting this value lower will make Linux issue
smaller discards and potentially help reduce latencies induced by large
discard operations.
discard_zeroes_data (RO)
------------------------
When read, this file will show if the discarded block are zeroed by the