Re: [Xen-users] Software Raid 5 domu performance drop

[braintorch <kkabardin@xxxxxxxxx>]

26.05.2013 15:51, James Harper ÐÐÑÐÑ:
> > -----Original Message-----
> > From: braintorch [mailto:kkabardin@xxxxxxxxx]
> > Sent: Sunday, 26 May 2013 9:49 PM
> > To: James Harper
> > Cc: xen-users@xxxxxxxxxxxxx
> > Subject: Re: [Xen-users] Software Raid 5 domu performance drop
> >
> > 26.05.2013 15:16, James Harper ÐÐÑÐÑ:
> > > > 26.05.2013 14:43, James Harper ÐÐÑÐÑ:
> > > > > > Hello. I'm experiencing solid I/O performance drop when using
> > software
> > > > > > raid 5 from PV DomU. I can get about 420M/s for sequential reads and
> > > > > > ~220 M/s for sequential writes when using it from Dom0. But it's only
> > > > > > ~170 M/s for read and ~80 M/s for write when using it from DomU.
> > > > > >
> > > > > > DomU performance for single drive is close to native â ~160 M/s for
> > > > > > reads and ~160 for writes.
> > > > > >
> > > > > > There is no filesystem or LVM, just raw data. Debian wheezy x86_64 for
> > > > > > both Dom0 and DomU. And "phy" backend is used to attach drive to
> > > > DomU.
> > > > > > Is this a bug or something wrong with my setup? What should I check?
> > > > > How are you measuring this performance?
> > > > >
> > > > > James
> > > > I  running dd if=/dev/zero of=/dev/xvdb bs=1M for several minutes.
> > > >
> > > > Also tried "cat /dev/zero | pv -r > /dev/xvdb " which gave me similar
> > > > results.
> > > Add oflag=direct to the dd command so that no caching is in effect and
> > then compare.
> > > James
> > James, it's even more dramatic without caching.
> >
> >
> > Dom0:
> >
> > Reading:
> > dd if=/dev/md0 of=/dev/null bs=1M iflag=direct
> >    ÑÐÐÐÐÑÐÐÐÐÐ 11659116544 ÐÐÐÑÐ (12 GB), 27,4614 c, 425 MB/c
> >
> > Writing:
> > dd if=/dev/zero of=/dev/md0 bs=1M oflag=direct
> >    ÑÐÐÐÐÑÐÐÐÐÐ 10108272640 ÐÐÐÑ (10 GB), 135,859 c, 74,4 MB/c
> >
> > Domu:
> >
> > Reading:
> > dd if=/dev/xvdb of=/dev/null iflag=direct
> >    ÑÐÐÐÐÑÐÐÐÐÐ 229615104 ÐÐÐÑÐ (230 MB), 75,9394 c, 3,0 MB/c
> >
> > Writing:
> > dd if=/dev/zero of=/dev/xvdb oflag=direct
> >    ÑÐÐÐÐÑÐÐÐÐÐ 231818240 ÐÐÐÑ (232 MB), 158,283 c, 1,5 MB/c
> I don't see a block size on the domu measurements... did you just copy and 
> paste it wrong or did you really leave it at default 512 byte block size?
>
> James
Ah, my mistake. I'm sorry. :(

Reading:
dd if=/dev/xvdb of=/dev/null bs=1M iflag=direct
 ÑÐÐÐÐÑÐÐÐÐÐ 13060014080 ÐÐÐÑ (13 GB), 58,949 c, 222 MB/c

Writing:
dd if=/dev/zero of=/dev/xvdb bs=1M oflag=direct
 ÑÐÐÐÐÑÐÐÐÐÐ 2241855488 ÐÐÐÑ (2,2 GB), 29,6292 c, 75,7 MB/c

So, writing without caching is almost the same. Reading is halfed in 
compare to dom0, but this is not really an issue to me.
Is there a way to optimize DomU caching to boost write speed?

Kirill.

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