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

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
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
Is this a bug or something wrong with my setup? What should I check?

How are you measuring this performance?

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
Add oflag=direct to the dd command so that no caching is in effect and
then compare.
James, it's even more dramatic without caching.


dd if=/dev/md0 of=/dev/null bs=1M iflag=direct
   ÑÐÐÐÐÑÐÐÐÐÐ 11659116544 ÐÐÐÑÐ (12 GB), 27,4614 c, 425 MB/c

dd if=/dev/zero of=/dev/md0 bs=1M oflag=direct
   ÑÐÐÐÐÑÐÐÐÐÐ 10108272640 ÐÐÐÑ (10 GB), 135,859 c, 74,4 MB/c


dd if=/dev/xvdb of=/dev/null iflag=direct
   ÑÐÐÐÐÑÐÐÐÐÐ 229615104 ÐÐÐÑÐ (230 MB), 75,9394 c, 3,0 MB/c

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?

Ah, my mistake. I'm sorry. :(

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

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?


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