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From: Avi Kivity <avi@redhat.com>
To: Kurt Yoder <ktykvm@yoderhome.com>
Cc: kvm@vger.kernel.org
Subject: Re: IO on guest is 20 times slower than host
Date: Wed, 25 Mar 2009 19:55:08 +0200	[thread overview]
Message-ID: <49CA6FFC.2030807@redhat.com> (raw)
In-Reply-To: <B947DF20-D97A-4D7C-9C3E-7064BEA0BC01@yoderhome.com>

Kurt Yoder wrote:
>
> What do you mean about the cache? Is my test fundamentally flawed? I 
> *thought* I was testing write speed on the disk...

'dd', without further arguments, will write to the page cache and let 
the kernel write the data back at a later time.  If you increase the 
block size (bs=1M count=1000) you should see much faster times on the 
host, I wouldn't be surprised to see 1GB/s.

If  you want to test disk speed, use of=/dev/blah oflag=direct.  Beware 
of destroying your data disk.

Something weird is happening with your system.  If you extend the test, 
what does 'top' show?  On both guest and host.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.


  reply	other threads:[~2009-03-25 17:54 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-03-25 15:23 IO on guest is 20 times slower than host Kurt Yoder
2009-03-25 16:13 ` Avi Kivity
2009-03-25 16:28   ` Kurt Yoder
2009-03-25 16:35     ` Avi Kivity
2009-03-25 16:51       ` Kurt Yoder
2009-03-25 16:54         ` Avi Kivity
2009-03-25 17:11           ` Kurt Yoder
2009-03-25 17:55             ` Avi Kivity [this message]
2009-03-25 18:51               ` Kurt Yoder
2009-03-26 10:16                 ` Avi Kivity
2009-03-27 13:53                   ` Kurt Yoder
2009-03-27 14:03                     ` Kurt Yoder
2009-03-27 14:10                       ` Kurt Yoder
2009-03-29 14:29                         ` Avi Kivity
2009-03-29 19:10                           ` Avi Kivity
2009-03-31  9:59                             ` Joerg Roedel
2009-03-31 10:02                               ` Avi Kivity
2009-04-04 11:47                               ` Avi Kivity
2009-03-30 19:39                           ` Kurt Yoder
2009-03-31 15:15                             ` Kurt Yoder

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