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awilliam
GitHub Repository: awilliam/linux-vfio
 Path: blob/master/Documentation/block/queue-sysfs.txt
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Queue sysfs files

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=================

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This text file will detail the queue files that are located in the sysfs tree

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for each block device. Note that stacked devices typically do not export

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any settings, since their queue merely functions are a remapping target.

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These files are the ones found in the /sys/block/xxx/queue/ directory.

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Files denoted with a RO postfix are readonly and the RW postfix means

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read-write.

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hw_sector_size (RO)

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-------------------

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This is the hardware sector size of the device, in bytes.

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max_hw_sectors_kb (RO)

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----------------------

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This is the maximum number of kilobytes supported in a single data transfer.

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max_sectors_kb (RW)

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-------------------

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This is the maximum number of kilobytes that the block layer will allow

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for a filesystem request. Must be smaller than or equal to the maximum

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size allowed by the hardware.

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nomerges (RW)

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-------------

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This enables the user to disable the lookup logic involved with IO

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merging requests in the block layer. By default (0) all merges are

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enabled. When set to 1 only simple one-hit merges will be tried. When

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set to 2 no merge algorithms will be tried (including one-hit or more

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complex tree/hash lookups).

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nr_requests (RW)

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----------------

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This controls how many requests may be allocated in the block layer for

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read or write requests. Note that the total allocated number may be twice

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this amount, since it applies only to reads or writes (not the accumulated

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sum).

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read_ahead_kb (RW)

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------------------

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Maximum number of kilobytes to read-ahead for filesystems on this block

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device.

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rq_affinity (RW)

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----------------

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If this option is enabled, the block layer will migrate request completions

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to the CPU that originally submitted the request. For some workloads

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this provides a significant reduction in CPU cycles due to caching effects.

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scheduler (RW)

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--------------

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When read, this file will display the current and available IO schedulers

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for this block device. The currently active IO scheduler will be enclosed

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in [] brackets. Writing an IO scheduler name to this file will switch

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control of this block device to that new IO scheduler. Note that writing

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an IO scheduler name to this file will attempt to load that IO scheduler

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module, if it isn't already present in the system.

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Jens Axboe <[email protected]>, February 2009

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