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commit bd8f7e6277 upstream.
If we have only 8K partial write at the beginning of a full RAID56
stripe, we will write the following contents:
0 8K 32K 64K
Disk 1 (data): |XX| | |
Disk 2 (data): | | |
Disk 3 (parity): |XXXXXXXXXXXXXXX|XXXXXXXXXXXXXXX|
|X| means the sector will be written back to disk.
Note that, although we won't write any sectors from disk 2, but we will
write the full 64KiB of parity to disk.
This behavior is fine for now, but not for the future (especially for
RAID56J, as we waste quite some space to journal the unused parity
stripes).
So here we will also utilize the btrfs_raid_bio::dbitmap, anytime we
queue a higher level bio into an rbio, we will update rbio::dbitmap to
indicate which vertical stripes we need to writeback.
And at finish_rmw(), we also check dbitmap to see if we need to write
any sector in the vertical stripe.
So after the patch, above example will only lead to the following
writeback pattern:
0 8K 32K 64K
Disk 1 (data): |XX| | |
Disk 2 (data): | | |
Disk 3 (parity): |XX| | |
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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.