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Re: [Xen-users] max loop xen 3.0.4

On Tue, Jan 16, 2007 at 10:50:31PM +0100, pcu-m@xxxxxxxxx wrote:
> On Tuesday 16 January 2007 17:56, Itamar Reis Peixoto wrote:
> > why you don't use tap:aio ?
> 
> Why don't you tell us what it is?
> 
> The Xen User's manual tells me (section A.2, disk parameter) to use 
> phy:UNAME,ioemu:DEV,MODE.
> 
> Somewhere else (section 6.2) it tells me to use something like 
> tap:aio:/full/path/to/vm1disk,sda1,w
> 
> Is the first deprecated and the second prefered?

You can use either.

However the tap:aio performs a lot better and more reliably in my
experience.

The loop version has a nasty tendency to run the kernel out of memory
when the block devices are busy.  This is to do with (as I understand
it) a lack of back pressure from output file side of the loop device
back to the block device.

The tap:aio is driven by a userspace program (tapdisk) which you can
set nice levels and thus IO priority levels on.

The one downside of tap:aio is that the block devices don't appear in
xm top so you can't see at a glance how much disk IO a domain is
doing.

I don't think you can use tap:aio with an HVM domain either.  At least
it didn't work when I tried it!

-- 
Nick Craig-Wood <nick@xxxxxxxxxxxxxx> -- http://www.craig-wood.com/nick

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