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dm: document sysfs entries Describe attributes provided by device-mapper in /sys/block. Signed-off-by: Milan Broz <mbroz@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
2012-03-29 01:41:23 +08:00
What: /sys/block/dm-<num>/dm/name
Date: January 2009
KernelVersion: 2.6.29
Contact: dm-devel@redhat.com
Description: Device-mapper device name.
Read-only string containing mapped device name.
Users: util-linux, device-mapper udev rules
What: /sys/block/dm-<num>/dm/uuid
Date: January 2009
KernelVersion: 2.6.29
Contact: dm-devel@redhat.com
Description: Device-mapper device UUID.
Read-only string containing DM-UUID or empty string
if DM-UUID is not set.
Users: util-linux, device-mapper udev rules
What: /sys/block/dm-<num>/dm/suspended
Date: June 2009
KernelVersion: 2.6.31
Contact: dm-devel@redhat.com
Description: Device-mapper device suspend state.
Contains the value 1 while the device is suspended.
Otherwise it contains 0. Read-only attribute.
Users: util-linux, device-mapper udev rules
dm: impose configurable deadline for dm_request_fn's merge heuristic Otherwise, for sequential workloads, the dm_request_fn can allow excessive request merging at the expense of increased service time. Add a per-device sysfs attribute to allow the user to control how long a request, that is a reasonable merge candidate, can be queued on the request queue. The resolution of this request dispatch deadline is in microseconds (ranging from 1 to 100000 usecs), to set a 20us deadline: echo 20 > /sys/block/dm-7/dm/rq_based_seq_io_merge_deadline The dm_request_fn's merge heuristic and associated extra accounting is disabled by default (rq_based_seq_io_merge_deadline is 0). This sysfs attribute is not applicable to bio-based DM devices so it will only ever report 0 for them. By allowing a request to remain on the queue it will block others requests on the queue. But introducing a short dequeue delay has proven very effective at enabling certain sequential IO workloads on really fast, yet IOPS constrained, devices to build up slightly larger IOs -- yielding 90+% throughput improvements. Having precise control over the time taken to wait for larger requests to build affords control beyond that of waiting for certain IO sizes to accumulate (which would require a deadline anyway). This knob will only ever make sense with sequential IO workloads and the particular value used is storage configuration specific. Given the expected niche use-case for when this knob is useful it has been deemed acceptable to expose this relatively crude method for crafting optimal IO on specific storage -- especially given the solution is simple yet effective. In the context of DM multipath, it is advisable to tune this sysfs attribute to a value that offers the best performance for the common case (e.g. if 4 paths are expected active, tune for that; if paths fail then performance may be slightly reduced). Alternatives were explored to have request-based DM autotune this value (e.g. if/when paths fail) but they were quickly deemed too fragile and complex to warrant further design and development time. If this problem proves more common as faster storage emerges we'll have to look at elevating a generic solution into the block core. Tested-by: Shiva Krishna Merla <shivakrishna.merla@netapp.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2015-02-26 13:50:28 +08:00
What: /sys/block/dm-<num>/dm/rq_based_seq_io_merge_deadline
Date: March 2015
KernelVersion: 4.1
Contact: dm-devel@redhat.com
Description: Allow control over how long a request that is a
reasonable merge candidate can be queued on the request
queue. The resolution of this deadline is in
microseconds (ranging from 1 to 100000 usecs).
Setting this attribute to 0 (the default) will disable
request-based DM's merge heuristic and associated extra
accounting. This attribute is not applicable to
bio-based DM devices so it will only ever report 0 for
them.