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

[Chris Unkel]

I don't offhand, but point well taken.  I suspect that showing a
difference in a benchmark is dependent on finding one where the
metadata has been evicted from the drive cache.  Let me think about it
a bit.

Thanks,

  --Chris


On Thu, Oct 22, 2020 at 10:42 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> 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
> >