| ================= | |
| 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 |