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When recoverying a single missing/failed device in a RAID6,
those stripes where the Q block is on the missing device are
handled a bit differently. In these cases it is easy to
check that the P block is correct, so we do. This results
in the P block be destroy. Consequently the P block needs
to be read a second time in order to compute Q. This causes
lots of seeks and hurts performance.
It shouldn't be necessary to re-read P as it can be computed
from the DATA. But we only compute blocks on missing
devices, since c337869d95 ("md: do not compute parity
unless it is on a failed drive").
So relax the change made in that commit to allow computing
of the P block in a RAID6 which it is the only missing that
block.
This makes RAID6 recovery run much faster as the disk just
"before" the recovering device is no longer seeking
back-and-forth.
Reported-by-tested-by: Brad Campbell <lists2009@fnarfbargle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.