Re: [RFC PATCH] ext4: fix 50% disk write performance regression

[Justin Maggard <jmaggard10@gmail.com>]

On Mon, Aug 30, 2010 at 5:37 PM, Ted Ts'o <tytso@mit.edu> wrote:
> On Mon, Aug 30, 2010 at 04:49:58PM -0400, Bill Fink wrote:
>> > Thanks for reporting it.  I'm going to have to take a closer look at
>> > why this makes a difference.  I'm going to guess though that what's
>> > going on is that we're posting writes in such a way that they're no
>> > longer aligned or ending at the end of a RAID5 stripe, causing a
>> > read-modify-write pass.  That would easily explain the write
>> > performance regression.
>>
>> I'm not sure I understand.  How could calling or not calling
>> ext4_num_dirty_pages() (unpatched versus patched 2.6.35 kernel)
>> affect the write alignment?
>
> Suppose you have 8 disks, with stripe size of 16k.  Assuming that
> you're only using one parity disk (i.e., RAID 5) and no spare disks,
> that means the optimal I/O size is 7*16k == 112k.  If we do a write
> which is smaller than 112k, or which is not a multiple of 112k, then
> the RAID subsystem will need to do a read-modify-write to update the
> parity disk.  Furthermore, the write had better be aligned on an 112k
> byte boundary.  The block allocator will guarantee that block #0 is
> aligned on a 112k block, but writes have to also be right size in
> order to avoid the read-modify-write.
>
> If we end up doing very small writes, then it can end up being quite
> disatrous for write performance.

I'd have to agree that this is likely the case.  Just to add a little
more data here, I tried the same 32GB dd test against a 12-disk MD
RAID 6 64k chunk array today with and without the patch (although
against a 2.6.33.7 kernel), and my write performance dropped from
~420MB/sec down to 350MB/sec when I used the patched kernel.

-Justin
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