Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 Queue sysfs files 2 ================= 3 4 This text file will detail the queue files that are located in the sysfs tree 5 for each block device. Note that stacked devices typically do not export 6 any settings, since their queue merely functions are a remapping target. 7 These files are the ones found in the /sys/block/xxx/queue/ directory. 8 9 Files denoted with a RO postfix are readonly and the RW postfix means 10 read-write. 11 12 add_random (RW) 13 ---------------- 14 This file allows to turn off the disk entropy contribution. Default 15 value of this file is '1'(on). 16 17 dax (RO) 18 -------- 19 This file indicates whether the device supports Direct Access (DAX), 20 used by CPU-addressable storage to bypass the pagecache. It shows '1' 21 if true, '0' if not. 22 23 discard_granularity (RO) 24 ----------------------- 25 This shows the size of internal allocation of the device in bytes, if 26 reported by the device. A value of '0' means device does not support 27 the discard functionality. 28 29 discard_max_hw_bytes (RO) 30 ---------------------- 31 Devices that support discard functionality may have internal limits on 32 the number of bytes that can be trimmed or unmapped in a single operation. 33 The discard_max_bytes parameter is set by the device driver to the maximum 34 number of bytes that can be discarded in a single operation. Discard 35 requests issued to the device must not exceed this limit. A discard_max_bytes 36 value of 0 means that the device does not support discard functionality. 37 38 discard_max_bytes (RW) 39 ---------------------- 40 While discard_max_hw_bytes is the hardware limit for the device, this 41 setting is the software limit. Some devices exhibit large latencies when 42 large discards are issued, setting this value lower will make Linux issue 43 smaller discards and potentially help reduce latencies induced by large 44 discard operations. 45 46 hw_sector_size (RO) 47 ------------------- 48 This is the hardware sector size of the device, in bytes. 49 50 io_poll (RW) 51 ------------ 52 When read, this file shows whether polling is enabled (1) or disabled 53 (0). Writing '0' to this file will disable polling for this device. 54 Writing any non-zero value will enable this feature. 55 56 io_poll_delay (RW) 57 ------------------ 58 If polling is enabled, this controls what kind of polling will be 59 performed. It defaults to -1, which is classic polling. In this mode, 60 the CPU will repeatedly ask for completions without giving up any time. 61 If set to 0, a hybrid polling mode is used, where the kernel will attempt 62 to make an educated guess at when the IO will complete. Based on this 63 guess, the kernel will put the process issuing IO to sleep for an amount 64 of time, before entering a classic poll loop. This mode might be a 65 little slower than pure classic polling, but it will be more efficient. 66 If set to a value larger than 0, the kernel will put the process issuing 67 IO to sleep for this amont of microseconds before entering classic 68 polling. 69 70 iostats (RW) 71 ------------- 72 This file is used to control (on/off) the iostats accounting of the 73 disk. 74 75 logical_block_size (RO) 76 ----------------------- 77 This is the logical block size of the device, in bytes. 78 79 max_hw_sectors_kb (RO) 80 ---------------------- 81 This is the maximum number of kilobytes supported in a single data transfer. 82 83 max_integrity_segments (RO) 84 --------------------------- 85 When read, this file shows the max limit of integrity segments as 86 set by block layer which a hardware controller can handle. 87 88 max_sectors_kb (RW) 89 ------------------- 90 This is the maximum number of kilobytes that the block layer will allow 91 for a filesystem request. Must be smaller than or equal to the maximum 92 size allowed by the hardware. 93 94 max_segments (RO) 95 ----------------- 96 Maximum number of segments of the device. 97 98 max_segment_size (RO) 99 --------------------- 100 Maximum segment size of the device. 101 102 minimum_io_size (RO) 103 -------------------- 104 This is the smallest preferred IO size reported by the device. 105 106 nomerges (RW) 107 ------------- 108 This enables the user to disable the lookup logic involved with IO 109 merging requests in the block layer. By default (0) all merges are 110 enabled. When set to 1 only simple one-hit merges will be tried. When 111 set to 2 no merge algorithms will be tried (including one-hit or more 112 complex tree/hash lookups). 113 114 nr_requests (RW) 115 ---------------- 116 This controls how many requests may be allocated in the block layer for 117 read or write requests. Note that the total allocated number may be twice 118 this amount, since it applies only to reads or writes (not the accumulated 119 sum). 120 121 To avoid priority inversion through request starvation, a request 122 queue maintains a separate request pool per each cgroup when 123 CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such 124 per-block-cgroup request pool. IOW, if there are N block cgroups, 125 each request queue may have up to N request pools, each independently 126 regulated by nr_requests. 127 128 optimal_io_size (RO) 129 -------------------- 130 This is the optimal IO size reported by the device. 131 132 physical_block_size (RO) 133 ------------------------ 134 This is the physical block size of device, in bytes. 135 136 read_ahead_kb (RW) 137 ------------------ 138 Maximum number of kilobytes to read-ahead for filesystems on this block 139 device. 140 141 rotational (RW) 142 --------------- 143 This file is used to stat if the device is of rotational type or 144 non-rotational type. 145 146 rq_affinity (RW) 147 ---------------- 148 If this option is '1', the block layer will migrate request completions to the 149 cpu "group" that originally submitted the request. For some workloads this 150 provides a significant reduction in CPU cycles due to caching effects. 151 152 For storage configurations that need to maximize distribution of completion 153 processing setting this option to '2' forces the completion to run on the 154 requesting cpu (bypassing the "group" aggregation logic). 155 156 scheduler (RW) 157 -------------- 158 When read, this file will display the current and available IO schedulers 159 for this block device. The currently active IO scheduler will be enclosed 160 in [] brackets. Writing an IO scheduler name to this file will switch 161 control of this block device to that new IO scheduler. Note that writing 162 an IO scheduler name to this file will attempt to load that IO scheduler 163 module, if it isn't already present in the system. 164 165 write_cache (RW) 166 ---------------- 167 When read, this file will display whether the device has write back 168 caching enabled or not. It will return "write back" for the former 169 case, and "write through" for the latter. Writing to this file can 170 change the kernels view of the device, but it doesn't alter the 171 device state. This means that it might not be safe to toggle the 172 setting from "write back" to "write through", since that will also 173 eliminate cache flushes issued by the kernel. 174 175 write_same_max_bytes (RO) 176 ------------------------- 177 This is the number of bytes the device can write in a single write-same 178 command. A value of '0' means write-same is not supported by this 179 device. 180 181 wb_lat_usec (RW) 182 ---------------- 183 If the device is registered for writeback throttling, then this file shows 184 the target minimum read latency. If this latency is exceeded in a given 185 window of time (see wb_window_usec), then the writeback throttling will start 186 scaling back writes. Writing a value of '0' to this file disables the 187 feature. Writing a value of '-1' to this file resets the value to the 188 default setting. 189 190 throttle_sample_time (RW) 191 ------------------------- 192 This is the time window that blk-throttle samples data, in millisecond. 193 blk-throttle makes decision based on the samplings. Lower time means cgroups 194 have more smooth throughput, but higher CPU overhead. This exists only when 195 CONFIG_BLK_DEV_THROTTLING_LOW is enabled. 196 197 Jens Axboe <jens.axboe@oracle.com>, February 2009