Re: MD write performance issue

[Asdo <asdo@shiftmail.org>]

mark delfman wrote:
> After further work we are sure that there is a significant write
> performance issue with either the Kernel+MD or...

Hm!
Pretty strange repeated ups and downs of the speed with increasing 
kernel versions.

Have you checked:
that compile options are the same (preferably by taking 2.6.31 compile 
options and porting them down)
disk schedulers are the same
the test was long enough to level jitters, like 2-3 minutes
Also: looking at "iostat -x 1" during the transfer could show something...

Apart from this, I confirm I noticed in my 2.6.31-rc? earlier tests, 
that performances on xfs writes were very inconsistent :
These were my benchmarks (I wrote them on file at that time):

Stripe_cache_size was 1024, 13 devices raid-5:

bs=1M -> 206MB/s
bs=256K -> 229MB/s

retrying soon after, identical settings:

bs=1M -> 129MB/s
bs=256K -> 140MB/s


Transfer speed was hence very unreliable, depending on something that is 
not clearly user visible... maybe dirty page cache? I thought that 
depending on the exact amount of data being pushed out by the pdflush at 
the first round, that would cause a sequence of read-modify-write stuff 
which would cause further read-modify-write and further instability 
later on. But I was doing that with raid-5 while you Mark are using 
raid-0 right? My theory doesn't hold on raid-0.