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