eb3b505366
Add sysfs interface to limit io during scrub. We relied on the ionice interface to do that, eg. the idle class let the system usable while scrub was running. This has changed when mq-deadline got widespread and did not implement the scheduling classes. That was a CFQ thing that got deleted. We've got numerous complaints from users about degraded performance. Currently only BFQ supports that but it's not a common scheduler and we can't ask everybody to switch to it. Alternatively the cgroup io limiting can be used but that also a non-trivial setup (v2 required, the controller must be enabled on the system). This can still be used if desired. Other ideas that have been explored: piggy-back on ionice (that is set per-process and is accessible) and interpret the class and classdata as bandwidth limits, but this does not have enough flexibility as there are only 8 allowed and we'd have to map fixed limits to each value. Also adjusting the value would need to lookup the process that currently runs scrub on the given device, and the value is not sticky so would have to be adjusted each time scrub runs. Running out of options, sysfs does not look that bad: - it's accessible from scripts, or udev rules - the name is similar to what MD-RAID has (/proc/sys/dev/raid/speed_limit_max or /sys/block/mdX/md/sync_speed_max) - the value is sticky at least for filesystem mount time - adjusting the value has immediate effect - sysfs is available in constrained environments (eg. system rescue) - the limit also applies to device replace Sysfs: - raw value is in bytes - values written to the file accept suffixes like K, M - file is in the per-device directory /sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max - 0 means use default priority of IO The scheduler is a simple deadline one and the accuracy is up to nearest 128K. Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.