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SubjectRe: [PATCH 0/3] mdraid sb and bitmap write alignment on 512e drives
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On 10/23/2020 11:31 AM, Christopher Unkel wrote:
> Hello all,
>
> While investigating some performance issues on mdraid 10 volumes
> formed with "512e" disks (4k native/physical sector size but with 512
> byte sector emulation), I've found two cases where mdraid will
> needlessly issue writes that start on 4k byte boundary, but are are
> shorter than 4k:
>
> 1. writes of the raid superblock; and
> 2. writes of the last page of the write-intent bitmap.
>
> The following is an excerpt of a blocktrace of one of the component
> members of a mdraid 10 volume during a 4k write near the end of the
> array:
>
> 8,32 11 2 0.000001687 711 D WS 2064 + 8 [kworker/11:1H]
> * 8,32 11 5 0.001454119 711 D WS 2056 + 1 [kworker/11:1H]
> * 8,32 11 8 0.002847204 711 D WS 2080 + 7 [kworker/11:1H]
> 8,32 11 11 0.003700545 3094 D WS 11721043920 + 8 [md127_raid1]
> 8,32 11 14 0.308785692 711 D WS 2064 + 8 [kworker/11:1H]
> * 8,32 11 17 0.310201697 711 D WS 2056 + 1 [kworker/11:1H]
> 8,32 11 20 5.500799245 711 D WS 2064 + 8 [kworker/11:1H]
> * 8,32 11 23 15.740923558 711 D WS 2080 + 7 [kworker/11:1H]
>
> Note the starred transactions, which each start on a 4k boundary, but
> are less than 4k in length, and so will use the 512-byte emulation.
> Sector 2056 holds the superblock, and is written as a single 512-byte
> write. Sector 2086 holds the bitmap bit relevant to the written
> sector. When it is written the active bits of the last page of the
> bitmap are written, starting at sector 2080, padded out to the end of
> the 512-byte logical sector as required. This results in a 3.5kb
> write, again using the 512-byte emulation.

Hi Christopher

Which superblock version do you use? If it's super1.1, superblock starts
at 0 sector.
If it's super1.2, superblock starts at 8 sector. If it's super1.0,
superblock starts at the
end of device and bitmap is before superblock. As mentioned above,
bitmap is behind
the superblock, so it should not be super1.0. So I have a question why
does 2056 hold
the superblock?

Regards
Xiao

>
> Note that in some arrays the last page of the bitmap may be
> sufficiently full that they are not affected by the issue with the
> bitmap write.
>
> As there can be a substantial penalty to using the 512-byte sector
> emulation (turning writes into read-modify writes if the relevant
> sector is not in the drive's cache) I believe it makes sense to pad
> these writes out to a 4k boundary. The writes are already padded out
> for "4k native" drives, where the short access is illegal.
>
> The following patch set changes the superblock and bitmap writes to
> respect the physical block size (e.g. 4k for today's 512e drives) when
> possible. In each case there is already logic for padding out to the
> underlying logical sector size. I reuse or repeat the logic for
> padding out to the physical sector size, but treat the padding out as
> optional rather than mandatory.
>
> The corresponding block trace with these patches is:
>
> 8,32 1 2 0.000003410 694 D WS 2064 + 8 [kworker/1:1H]
> 8,32 1 5 0.001368788 694 D WS 2056 + 8 [kworker/1:1H]
> 8,32 1 8 0.002727981 694 D WS 2080 + 8 [kworker/1:1H]
> 8,32 1 11 0.003533831 3063 D WS 11721043920 + 8 [md127_raid1]
> 8,32 1 14 0.253952321 694 D WS 2064 + 8 [kworker/1:1H]
> 8,32 1 17 0.255354215 694 D WS 2056 + 8 [kworker/1:1H]
> 8,32 1 20 5.337938486 694 D WS 2064 + 8 [kworker/1:1H]
> 8,32 1 23 15.577963062 694 D WS 2080 + 8 [kworker/1:1H]
>
> I do notice that the code for bitmap writes has a more sophisticated
> and thorough check for overlap than the code for superblock writes.
> (Compare write_sb_page in md-bitmap.c vs. super_1_load in md.c.) From
> what I know since the various structures starts have always been 4k
> aligned anyway, it is always safe to pad the superblock write out to
> 4k (as occurs on 4k native drives) but not necessarily futher.
>
> Feedback appreciated.
>
> --Chris
>
>
> Christopher Unkel (3):
> md: align superblock writes to physical blocks
> md: factor sb write alignment check into function
> md: pad writes to end of bitmap to physical blocks
>
> drivers/md/md-bitmap.c | 80 +++++++++++++++++++++++++-----------------
> drivers/md/md.c | 15 ++++++++
> 2 files changed, 63 insertions(+), 32 deletions(-)
>

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