1Queue sysfs files 2================= 3 4This text file will detail the queue files that are located in the sysfs tree 5for each block device. Note that stacked devices typically do not export 6any settings, since their queue merely functions are a remapping target. 7These files are the ones found in the /sys/block/xxx/queue/ directory. 8 9Files denoted with a RO postfix are readonly and the RW postfix means 10read-write. 11 12hw_sector_size (RO) 13------------------- 14This is the hardware sector size of the device, in bytes. 15 16max_hw_sectors_kb (RO) 17---------------------- 18This is the maximum number of kilobytes supported in a single data transfer. 19 20max_sectors_kb (RW) 21------------------- 22This is the maximum number of kilobytes that the block layer will allow 23for a filesystem request. Must be smaller than or equal to the maximum 24size allowed by the hardware. 25 26nomerges (RW) 27------------- 28This enables the user to disable the lookup logic involved with IO merging 29requests in the block layer. Merging may still occur through a direct 301-hit cache, since that comes for (almost) free. The IO scheduler will not 31waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults 32to 0, enabling all merges. 33 34nr_requests (RW) 35---------------- 36This controls how many requests may be allocated in the block layer for 37read or write requests. Note that the total allocated number may be twice 38this amount, since it applies only to reads or writes (not the accumulated 39sum). 40 41read_ahead_kb (RW) 42------------------ 43Maximum number of kilobytes to read-ahead for filesystems on this block 44device. 45 46rq_affinity (RW) 47---------------- 48If this option is enabled, the block layer will migrate request completions 49to the CPU that originally submitted the request. For some workloads 50this provides a significant reduction in CPU cycles due to caching effects. 51 52scheduler (RW) 53-------------- 54When read, this file will display the current and available IO schedulers 55for this block device. The currently active IO scheduler will be enclosed 56in [] brackets. Writing an IO scheduler name to this file will switch 57control of this block device to that new IO scheduler. Note that writing 58an IO scheduler name to this file will attempt to load that IO scheduler 59module, if it isn't already present in the system. 60 61 62 63Jens Axboe <jens.axboe@oracle.com>, February 2009 64