Re: [PATCH 0/3] mdraid sb and bitmap write alignment on 512e drives

[Song Liu]

On Thu, Oct 22, 2020 at 8:31 PM Christopher Unkel <cunkel@xxxxxxxxxxxxxx> 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.
>
> 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

Thanks for the patches. Do you have performance numbers before/after these
changes? Some micro benchmarks results would be great motivation.

Thanks,
Song


>
>
> 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(-)
>
> --
> 2.17.1
>