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diff --git a/Documentation/block/queue-sysfs.rst b/Documentation/block/queue-sysfs.rst new file mode 100644 index 000000000000..6a8513af9201 --- /dev/null +++ b/Documentation/block/queue-sysfs.rst @@ -0,0 +1,254 @@ +================= +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). + +chunk_sectors (RO) +------------------ +This has different meaning depending on the type of the block device. +For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors +of the RAID volume stripe segment. For a zoned block device, either host-aware +or host-managed, chunk_sectors indicates the size in 512B sectors of the zones +of the device, with the eventual exception of the last zone of the device which +may be smaller. + +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. + +discard_zeroes_data (RO) +------------------------ +Obsolete. Always zero. + +fua (RO) +-------- +Whether or not the block driver supports the FUA flag for write requests. +FUA stands for Force Unit Access. If the FUA flag is set that means that +write requests must bypass the volatile cache of the storage device. + +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 amount of microseconds before entering classic +polling. + +io_timeout (RW) +--------------- +io_timeout is the request timeout in milliseconds. If a request does not +complete in this time then the block driver timeout handler is invoked. +That timeout handler can decide to retry the request, to fail it or to start +a device recovery strategy. + +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_discard_segments (RO) +------------------------- +The maximum number of DMA scatter/gather entries in a discard request. + +max_hw_sectors_kb (RO) +---------------------- +This is the maximum number of kilobytes supported in a single data transfer. + +max_integrity_segments (RO) +--------------------------- +Maximum number of elements in a DMA scatter/gather list with integrity +data that will be submitted by the block layer core to the associated +block driver. + +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 elements in a DMA scatter/gather list that is submitted +to the associated block driver. + +max_segment_size (RO) +--------------------- +Maximum size in bytes of a single element in a DMA scatter/gather list. + +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. + +nr_zones (RO) +------------- +For zoned block devices (zoned attribute indicating "host-managed" or +"host-aware"), this indicates the total number of zones of the device. +This is always 0 for regular block devices. + +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. + +wbt_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. + +write_zeroes_max_bytes (RO) +--------------------------- +For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of +bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES +is not supported. + +zoned (RO) +---------- +This indicates if the device is a zoned block device and the zone model of the +device if it is indeed zoned. The possible values indicated by zoned are +"none" for regular block devices and "host-aware" or "host-managed" for zoned +block devices. The characteristics of host-aware and host-managed zoned block +devices are described in the ZBC (Zoned Block Commands) and ZAC +(Zoned Device ATA Command Set) standards. These standards also define the +"drive-managed" zone model. However, since drive-managed zoned block devices +do not support zone commands, they will be treated as regular block devices +and zoned will report "none". + +Jens Axboe <jens.axboe@oracle.com>, February 2009 |