[Xen-devel] slow disk response

[Adam Heath <doogie@xxxxxxxxxxxxx>]

I've been attempting to improve the disk response we've been seeing on our
hosts.  I've seen some very odd numbers, and can't explain them.

I've got enbd on my dom0, as well as a local disk.  The enbd I have configured
to export a device, then import it again on localhost.  I get 20MB/s from this
setup.

>From the real disk, I get 55MB/s.

For comparison, I also imported an enbd over the network.  I get the natural
speed of 10MB/s from that.

Then, I imported them into a domU instance.  This is where things get really
odd.

The enbd over localhost gets 19MB/s.  This is acceptable.

The enbd over the network gets 9.5MB/s.  This is also acceptable.

However, the emulated disk only gets 12.5MB/s.

I'm using defaults everywhere.  Xen version 2.0.6, linux 2.6.10.

ps: Please, don't mention that I should upgrade xen to unstable; that's not
an acceptable option.  I want to know why real disks are somehow getting
treated specially;  I want an explanation for this problem.

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