Queue sysfs files================= This text file will detail the queue files that are located in the sysfs treefor each block device. Note that stacked devices typically do not exportany 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 meansread-write. hw_sector_size (RO)-------------------This is the hardware sector size of the device, in bytes. max_hw_sectors_kb (RO)----------------------This is the maximum number of kilobytes supported in a single data transfer. max_sectors_kb (RW)-------------------This is the maximum number of kilobytes that the block layer will allowfor a filesystem request. Must be smaller than or equal to the maximumsize allowed by the hardware. nomerges (RW)-------------This enables the user to disable the lookup logic involved with IOmerging requests in the block layer. By default (0) all merges areenabled. When set to 1 only simple one-hit merges will be tried. Whenset to 2 no merge algorithms will be tried (including one-hit or morecomplex tree/hash lookups). nr_requests (RW)----------------This controls how many requests may be allocated in the block layer forread or write requests. Note that the total allocated number may be twicethis amount, since it applies only to reads or writes (not the accumulatedsum). read_ahead_kb (RW)------------------Maximum number of kilobytes to read-ahead for filesystems on this blockdevice. rq_affinity (RW)----------------If this option is '1', the block layer will migrate request completions to thecpu "group" that originally submitted the request. For some workloads thisprovides a significant reduction in CPU cycles due to caching effects. For storage configurations that need to maximize distribution of completionprocessing setting this option to '2' forces the completion to run on therequesting cpu (bypassing the "group" aggregation logic). scheduler (RW)--------------When read, this file will display the current and available IO schedulersfor this block device. The currently active IO scheduler will be enclosedin [] brackets. Writing an IO scheduler name to this file will switchcontrol of this block device to that new IO scheduler. Note that writingan IO scheduler name to this file will attempt to load that IO schedulermodule, if it isn't already present in the system.   Jens Axboe <jens.axboe@oracle.com>, February 2009