290 lines
11 KiB
ReStructuredText
290 lines
11 KiB
ReStructuredText
=================
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Queue sysfs files
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=================
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This text file will detail the queue files that are located in the sysfs tree
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for each block device. Note that stacked devices typically do not export
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any settings, since their queue merely functions are a remapping target.
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These files are the ones found in the /sys/block/xxx/queue/ directory.
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Files denoted with a RO postfix are readonly and the RW postfix means
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read-write.
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add_random (RW)
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---------------
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This file allows to turn off the disk entropy contribution. Default
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value of this file is '1'(on).
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chunk_sectors (RO)
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------------------
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This has different meaning depending on the type of the block device.
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For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors
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of the RAID volume stripe segment. For a zoned block device, either host-aware
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or host-managed, chunk_sectors indicates the size in 512B sectors of the zones
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of the device, with the eventual exception of the last zone of the device which
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may be smaller.
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dax (RO)
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--------
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This file indicates whether the device supports Direct Access (DAX),
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used by CPU-addressable storage to bypass the pagecache. It shows '1'
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if true, '0' if not.
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discard_granularity (RO)
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------------------------
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This shows the size of internal allocation of the device in bytes, if
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reported by the device. A value of '0' means device does not support
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the discard functionality.
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discard_max_hw_bytes (RO)
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-------------------------
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Devices that support discard functionality may have internal limits on
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the number of bytes that can be trimmed or unmapped in a single operation.
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The discard_max_bytes parameter is set by the device driver to the maximum
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number of bytes that can be discarded in a single operation. Discard
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requests issued to the device must not exceed this limit. A discard_max_bytes
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value of 0 means that the device does not support discard functionality.
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discard_max_bytes (RW)
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----------------------
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While discard_max_hw_bytes is the hardware limit for the device, this
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setting is the software limit. Some devices exhibit large latencies when
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large discards are issued, setting this value lower will make Linux issue
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smaller discards and potentially help reduce latencies induced by large
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discard operations.
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discard_zeroes_data (RO)
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------------------------
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Obsolete. Always zero.
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fua (RO)
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--------
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Whether or not the block driver supports the FUA flag for write requests.
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FUA stands for Force Unit Access. If the FUA flag is set that means that
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write requests must bypass the volatile cache of the storage device.
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hw_sector_size (RO)
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-------------------
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This is the hardware sector size of the device, in bytes.
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io_poll (RW)
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------------
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When read, this file shows whether polling is enabled (1) or disabled
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(0). Writing '0' to this file will disable polling for this device.
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Writing any non-zero value will enable this feature.
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io_poll_delay (RW)
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------------------
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If polling is enabled, this controls what kind of polling will be
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performed. It defaults to -1, which is classic polling. In this mode,
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the CPU will repeatedly ask for completions without giving up any time.
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If set to 0, a hybrid polling mode is used, where the kernel will attempt
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to make an educated guess at when the IO will complete. Based on this
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guess, the kernel will put the process issuing IO to sleep for an amount
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of time, before entering a classic poll loop. This mode might be a
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little slower than pure classic polling, but it will be more efficient.
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If set to a value larger than 0, the kernel will put the process issuing
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IO to sleep for this amount of microseconds before entering classic
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polling.
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io_timeout (RW)
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---------------
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io_timeout is the request timeout in milliseconds. If a request does not
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complete in this time then the block driver timeout handler is invoked.
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That timeout handler can decide to retry the request, to fail it or to start
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a device recovery strategy.
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iostats (RW)
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-------------
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This file is used to control (on/off) the iostats accounting of the
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disk.
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logical_block_size (RO)
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-----------------------
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This is the logical block size of the device, in bytes.
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max_discard_segments (RO)
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-------------------------
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The maximum number of DMA scatter/gather entries in a discard request.
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max_hw_sectors_kb (RO)
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----------------------
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This is the maximum number of kilobytes supported in a single data transfer.
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max_integrity_segments (RO)
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---------------------------
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Maximum number of elements in a DMA scatter/gather list with integrity
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data that will be submitted by the block layer core to the associated
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block driver.
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max_active_zones (RO)
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---------------------
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For zoned block devices (zoned attribute indicating "host-managed" or
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"host-aware"), the sum of zones belonging to any of the zone states:
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EXPLICIT OPEN, IMPLICIT OPEN or CLOSED, is limited by this value.
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If this value is 0, there is no limit.
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If the host attempts to exceed this limit, the driver should report this error
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with BLK_STS_ZONE_ACTIVE_RESOURCE, which user space may see as the EOVERFLOW
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errno.
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max_open_zones (RO)
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-------------------
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For zoned block devices (zoned attribute indicating "host-managed" or
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"host-aware"), the sum of zones belonging to any of the zone states:
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EXPLICIT OPEN or IMPLICIT OPEN, is limited by this value.
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If this value is 0, there is no limit.
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If the host attempts to exceed this limit, the driver should report this error
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with BLK_STS_ZONE_OPEN_RESOURCE, which user space may see as the ETOOMANYREFS
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errno.
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max_sectors_kb (RW)
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-------------------
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This is the maximum number of kilobytes that the block layer will allow
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for a filesystem request. Must be smaller than or equal to the maximum
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size allowed by the hardware.
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max_segments (RO)
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-----------------
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Maximum number of elements in a DMA scatter/gather list that is submitted
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to the associated block driver.
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max_segment_size (RO)
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---------------------
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Maximum size in bytes of a single element in a DMA scatter/gather list.
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minimum_io_size (RO)
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--------------------
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This is the smallest preferred IO size reported by the device.
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nomerges (RW)
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-------------
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This enables the user to disable the lookup logic involved with IO
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merging requests in the block layer. By default (0) all merges are
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enabled. When set to 1 only simple one-hit merges will be tried. When
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set to 2 no merge algorithms will be tried (including one-hit or more
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complex tree/hash lookups).
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nr_requests (RW)
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----------------
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This controls how many requests may be allocated in the block layer for
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read or write requests. Note that the total allocated number may be twice
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this amount, since it applies only to reads or writes (not the accumulated
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sum).
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To avoid priority inversion through request starvation, a request
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queue maintains a separate request pool per each cgroup when
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CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
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per-block-cgroup request pool. IOW, if there are N block cgroups,
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each request queue may have up to N request pools, each independently
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regulated by nr_requests.
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nr_zones (RO)
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-------------
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For zoned block devices (zoned attribute indicating "host-managed" or
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"host-aware"), this indicates the total number of zones of the device.
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This is always 0 for regular block devices.
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optimal_io_size (RO)
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--------------------
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This is the optimal IO size reported by the device.
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physical_block_size (RO)
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------------------------
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This is the physical block size of device, in bytes.
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read_ahead_kb (RW)
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------------------
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Maximum number of kilobytes to read-ahead for filesystems on this block
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device.
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rotational (RW)
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---------------
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This file is used to stat if the device is of rotational type or
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non-rotational type.
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rq_affinity (RW)
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----------------
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If this option is '1', the block layer will migrate request completions to the
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cpu "group" that originally submitted the request. For some workloads this
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provides a significant reduction in CPU cycles due to caching effects.
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For storage configurations that need to maximize distribution of completion
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processing setting this option to '2' forces the completion to run on the
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requesting cpu (bypassing the "group" aggregation logic).
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scheduler (RW)
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--------------
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When read, this file will display the current and available IO schedulers
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for this block device. The currently active IO scheduler will be enclosed
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in [] brackets. Writing an IO scheduler name to this file will switch
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control of this block device to that new IO scheduler. Note that writing
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an IO scheduler name to this file will attempt to load that IO scheduler
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module, if it isn't already present in the system.
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write_cache (RW)
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----------------
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When read, this file will display whether the device has write back
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caching enabled or not. It will return "write back" for the former
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case, and "write through" for the latter. Writing to this file can
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change the kernels view of the device, but it doesn't alter the
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device state. This means that it might not be safe to toggle the
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setting from "write back" to "write through", since that will also
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eliminate cache flushes issued by the kernel.
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write_same_max_bytes (RO)
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-------------------------
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This is the number of bytes the device can write in a single write-same
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command. A value of '0' means write-same is not supported by this
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device.
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wbt_lat_usec (RW)
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-----------------
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If the device is registered for writeback throttling, then this file shows
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the target minimum read latency. If this latency is exceeded in a given
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window of time (see wb_window_usec), then the writeback throttling will start
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scaling back writes. Writing a value of '0' to this file disables the
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feature. Writing a value of '-1' to this file resets the value to the
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default setting.
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throttle_sample_time (RW)
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-------------------------
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This is the time window that blk-throttle samples data, in millisecond.
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blk-throttle makes decision based on the samplings. Lower time means cgroups
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have more smooth throughput, but higher CPU overhead. This exists only when
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CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
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write_zeroes_max_bytes (RO)
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---------------------------
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For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of
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bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES
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is not supported.
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zone_append_max_bytes (RO)
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--------------------------
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This is the maximum number of bytes that can be written to a sequential
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zone of a zoned block device using a zone append write operation
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(REQ_OP_ZONE_APPEND). This value is always 0 for regular block devices.
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zoned (RO)
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----------
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This indicates if the device is a zoned block device and the zone model of the
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device if it is indeed zoned. The possible values indicated by zoned are
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"none" for regular block devices and "host-aware" or "host-managed" for zoned
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block devices. The characteristics of host-aware and host-managed zoned block
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devices are described in the ZBC (Zoned Block Commands) and ZAC
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(Zoned Device ATA Command Set) standards. These standards also define the
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"drive-managed" zone model. However, since drive-managed zoned block devices
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do not support zone commands, they will be treated as regular block devices
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and zoned will report "none".
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zone_write_granularity (RO)
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---------------------------
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This indicates the alignment constraint, in bytes, for write operations in
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sequential zones of zoned block devices (devices with a zoned attributed
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that reports "host-managed" or "host-aware"). This value is always 0 for
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regular block devices.
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Jens Axboe <jens.axboe@oracle.com>, February 2009
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