[PATCH v2 0/3] btrfs: raid56: reduce unnecessary parity writes

[Qu Wenruo <wqu@suse.com>]

[Changelog]
v2:
- Cleanup the data bitmap after all the IO of the full stripe finished
  This is to ensure a cached rbio will not put its old dbitmap for
  the next one whoe uses the cache.
  (only for the last patch)

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.

This is due to the fact that we don't really check if the vertical
stripe has any data (aka, range from higher level bio) for parity
stripes.

The patchset will convert it to the following write pattern:

                    0  8K           32K             64K
Disk 1	(data):     |XX|            |               |
Disk 2  (data):     |               |               |
Disk 3  (parity):   |XX|            |               |

This is done by fully utilize btrfs_raid_bio::dbitmap which is only
utilized by scrub path.

Now write path (either partial or full write) will also populate
btrfs_raid_bio::dbitmap, and then only assemble sectors marked in
dbitmap for submission.

The first two patches is just to make previous bitmap pointers into
integrated bitmaps inside the bbtrfs_raid_bio and scrub_parity.

This saves 8 bytes for each structure.

The final patch does the most important work, by introducing a new
helper, rbio_add_bio() to mark the btrfs_raid_bio::dbitmap.
Then in finish_rmw() only add sectors which has bit in dbitmap marked to
change the write pattern.


Qu Wenruo (3):
  btrfs: use integrated bitmaps for btrfs_raid_bio::dbitmap and
    finish_pbitmap
  btrfs: use integrated bitmaps for scrub_parity::dbitmap and ebitmap
  btrfs: only write the sectors in the vertical stripe which has data
    stripes

 fs/btrfs/raid56.c | 95 +++++++++++++++++++++++++++++++++++------------
 fs/btrfs/scrub.c  | 32 +++++++---------
 2 files changed, 84 insertions(+), 43 deletions(-)

-- 
2.36.1