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RE: [Xen-devel] Re: poor domU VBD performance.

To: "peter bier" <peter_bier@xxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Re: poor domU VBD performance.
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 29 Mar 2005 14:38:22 +0100
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Thread-index: AcU0VGRzbyp7PVQUTGSo4lNq3twk0gAC/Tcw
Thread-topic: [Xen-devel] Re: poor domU VBD performance.
> "dd if=/dev/hde1 of=/dev/null bs=1024k count=1024"
> in domU. 
> hdparm told that the default setup was 256k readahead.

Do you mean KB or sectors?

> I have tested the performance with the following readahead settings:
> readahead    |     duration 
> 128 sectors  |     160 sec
> 256 sectors  |      76 sec
> 512 sectors  |      18.5 sec
> 1024 sectors |      19.5 sec
> 1200 sectors |     457 sec
> dom0 takes 18.0 secs no matter of the readahead setting in Dom0 is.

Would you mind repeating these experiments with a 2.4 dom0 and a 2.6domU

The performance cliff below 512 and above 1024 sectors is spectacular.
This is all rather confusing, but at least we know it can be made to
work fast. Changing the domU readahead is unlikely to be the right fix.
We just need to figure out how to keep it on the sweet spot...

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