[PATCH md-6.13 0/5] md/md-bitmap: move bitmap_{start,end}write to md upper layer

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

While reviewing raid5 code, it's found that bitmap operations can be
optimized. For example, for a 4 disks raid5, with chunksize=8k, if user
issue a IO (0 + 48k) to the array:

┌────────────────────────────────────────────────────────────┐
│chunk 0                                                     │
│      ┌────────────┬─────────────┬─────────────┬────────────┼
│  sh0 │A0: 0 + 4k  │A1: 8k + 4k  │A2: 16k + 4k │A3: P       │
│      ┼────────────┼─────────────┼─────────────┼────────────┼
│  sh1 │B0: 4k + 4k │B1: 12k + 4k │B2: 20k + 4k │B3: P       │
┼──────┴────────────┴─────────────┴─────────────┴────────────┼
│chunk 1                                                     │
│      ┌────────────┬─────────────┬─────────────┬────────────┤
│  sh2 │C0: 24k + 4k│C1: 32k + 4k │C2: P        │C3: 40k + 4k│
│      ┼────────────┼─────────────┼─────────────┼────────────┼
│  sh3 │D0: 28k + 4k│D1: 36k + 4k │D2: P        │D3: 44k + 4k│
└──────┴────────────┴─────────────┴─────────────┴────────────┘

Before this set, 4 stripe head will be used, and each sh will attach
bio for 3 disks, and each attached bio will trigger
bitmap_startwrite() once, which means total 12 times.
 - 3 times (0 + 4k), for (A0, A1 and A2)
 - 3 times (4 + 4k), for (B0, B1 and B2)
 - 3 times (8 + 4k), for (C0, C1 and C3)
 - 3 times (12 + 4k), for (D0, D1 and D3)

After this set, md upper layer will calculate that IO range (0 + 48k)
is corresponding to the bitmap (0 + 16k), and call bitmap_startwrite()
just once.

Noted that this patch will align bitmap ranges to the chunks, for example,
if user issue a IO (0 + 4k) to array:

- Before this set, 1 time (0 + 4k), for A0;
- After this set, 1 time (0 + 8k) for chunk 0;

Usually, one bitmap bit will represent more than one disk chunk, and this
doesn't have any difference. And even if user really created a array
that one chunk contain multiple bits, the overhead is that more data
will be recovered after power failure.

Yu Kuai (5):
  md/md-bitmap: factor behind write counters out from
    bitmap_{start/end}write()
  md/md-bitmap: remove the last parameter for bimtap_ops->endwrite()
  md: add a new callback pers->bitmap_sector()
  md/raid5: implement pers->bitmap_sector()
  md/md-bitmap: move bitmap_{start, end}write to md upper layer

 drivers/md/md-bitmap.c   | 74 ++++++++++++++++++++++++----------------
 drivers/md/md-bitmap.h   |  7 ++--
 drivers/md/md.c          | 29 ++++++++++++++++
 drivers/md/md.h          |  5 +++
 drivers/md/raid1.c       | 11 +++---
 drivers/md/raid10.c      |  6 ----
 drivers/md/raid5-cache.c |  4 ---
 drivers/md/raid5.c       | 48 ++++++++++++--------------
 8 files changed, 109 insertions(+), 75 deletions(-)

-- 
2.39.2