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Queue sysfs files
=================

This text file will detail the queue files that are located in the sysfs tree
for each block device. Note that stacked devices typically do not export
any settings, since their queue merely functions are a remapping target.
These files are the ones found in the /sys/block/xxx/queue/ directory.

Files denoted with a RO postfix are readonly and the RW postfix means
read-write.

add_random (RW)
----------------
This file allows to turn off the disk entropy contribution. Default
value of this file is '1'(on).

dax (RO)
--------
This file indicates whether the device supports Direct Access (DAX),
used by CPU-addressable storage to bypass the pagecache.  It shows '1'
if true, '0' if not.

discard_granularity (RO)
-----------------------
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_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.
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.

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.

hw_sector_size (RO)
-------------------
This is the hardware sector size of the device, in bytes.

io_poll (RW)
------------
When read, this file shows whether polling is enabled (1) or disabled
(0).  Writing '0' to this file will disable polling for this device.
Writing any non-zero value will enable this feature.

io_poll_delay (RW)
------------------
If polling is enabled, this controls what kind of polling will be
performed. It defaults to -1, which is classic polling. In this mode,
the CPU will repeatedly ask for completions without giving up any time.
If set to 0, a hybrid polling mode is used, where the kernel will attempt
to make an educated guess at when the IO will complete. Based on this
guess, the kernel will put the process issuing IO to sleep for an amount
of time, before entering a classic poll loop. This mode might be a
little slower than pure classic polling, but it will be more efficient.
If set to a value larger than 0, the kernel will put the process issuing
IO to sleep for this amont of microseconds before entering classic
polling.

iostats (RW)
-------------
This file is used to control (on/off) the iostats accounting of the
disk.

logical_block_size (RO)
-----------------------
This is the logical block size of the device, in bytes.

max_hw_sectors_kb (RO)
----------------------
This is the maximum number of kilobytes supported in a single data transfer.

max_integrity_segments (RO)
---------------------------
When read, this file shows the max limit of integrity segments as
set by block layer which a hardware controller can handle.

max_sectors_kb (RW)
-------------------
This is the maximum number of kilobytes that the block layer will allow
for a filesystem request. Must be smaller than or equal to the maximum
size allowed by the hardware.

max_segments (RO)
-----------------
Maximum number of segments of the device.

max_segment_size (RO)
---------------------
Maximum segment size of the device.

minimum_io_size (RO)
--------------------
This is the smallest preferred IO size reported by the device.

nomerges (RW)
-------------
This enables the user to disable the lookup logic involved with IO
merging requests in the block layer. By default (0) all merges are
enabled. When set to 1 only simple one-hit merges will be tried. When
set to 2 no merge algorithms will be tried (including one-hit or more
complex tree/hash lookups).

nr_requests (RW)
----------------
This controls how many requests may be allocated in the block layer for
read or write requests. Note that the total allocated number may be twice
this amount, since it applies only to reads or writes (not the accumulated
sum).

To avoid priority inversion through request starvation, a request
queue maintains a separate request pool per each cgroup when
CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
per-block-cgroup request pool.  IOW, if there are N block cgroups,
each request queue may have up to N request pools, each independently
regulated by nr_requests.

optimal_io_size (RO)
--------------------
This is the optimal IO size reported by the device.

physical_block_size (RO)
------------------------
This is the physical block size of device, in bytes.

read_ahead_kb (RW)
------------------
Maximum number of kilobytes to read-ahead for filesystems on this block
device.

rotational (RW)
---------------
This file is used to stat if the device is of rotational type or
non-rotational type.

rq_affinity (RW)
----------------
If this option is '1', the block layer will migrate request completions to the
cpu "group" that originally submitted the request. For some workloads this
provides a significant reduction in CPU cycles due to caching effects.

For storage configurations that need to maximize distribution of completion
processing setting this option to '2' forces the completion to run on the
requesting cpu (bypassing the "group" aggregation logic).

scheduler (RW)
--------------
When read, this file will display the current and available IO schedulers
for this block device. The currently active IO scheduler will be enclosed
in [] brackets. Writing an IO scheduler name to this file will switch
control of this block device to that new IO scheduler. Note that writing
an IO scheduler name to this file will attempt to load that IO scheduler
module, if it isn't already present in the system.

write_cache (RW)
----------------
When read, this file will display whether the device has write back
caching enabled or not. It will return "write back" for the former
case, and "write through" for the latter. Writing to this file can
change the kernels view of the device, but it doesn't alter the
device state. This means that it might not be safe to toggle the
setting from "write back" to "write through", since that will also
eliminate cache flushes issued by the kernel.

write_same_max_bytes (RO)
-------------------------
This is the number of bytes the device can write in a single write-same
command.  A value of '0' means write-same is not supported by this
device.

wb_lat_usec (RW)
----------------
If the device is registered for writeback throttling, then this file shows
the target minimum read latency. If this latency is exceeded in a given
window of time (see wb_window_usec), then the writeback throttling will start
scaling back writes. Writing a value of '0' to this file disables the
feature. Writing a value of '-1' to this file resets the value to the
default setting.

throttle_sample_time (RW)
-------------------------
This is the time window that blk-throttle samples data, in millisecond.
blk-throttle makes decision based on the samplings. Lower time means cgroups
have more smooth throughput, but higher CPU overhead. This exists only when
CONFIG_BLK_DEV_THROTTLING_LOW is enabled.

Jens Axboe <jens.axboe@oracle.com>, February 2009