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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