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Re: [Xen-users] Slow Disk access on SLES 10 install

To: D Sims <dwains@xxxxxxxxx>
Subject: Re: [Xen-users] Slow Disk access on SLES 10 install
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Mon, 5 Mar 2007 17:50:24 +0000
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On Sun, Mar 04, 2007 at 05:57:30PM -0800, D Sims wrote:
> 
> Matthias:
> 
> You were 100% correct.  That was the piece of information that I did not
> know that I needed.  As soon as I changed the setting in the
> /etc/xen/scripts/block  script from
> 
> do_or_die losetup -y "$loopdev" "$file"
> 
> do_or_die losetup  "$loopdev" "$file"
> 
> this made all the difference in the world.  
> 
> However, this does beg another question. What is the best practice for
> setting up a virtual disk for a guest domain?
> 
> Is it better to live with the possibility of data corruption on a host crash
> and have normal disk access, or turn the flag on and be protected against
> corruption, but suffer terrible performance.  Or is there a better way? 

Don't use 'file:'  at all - loopback devices are just not a viable storage
backend due to the horrific performance vs data integrity tradeoff you have
to make. A physical disk device is the best option, but failing that use 
blktap for file based disks. eg use the tap:aio:/var/lib/xen/images/foo.img
URL scheme for the disk.  NB, if you want good performance with file based
images, regardless of whether using blktap or loopback one should pre-allocate
the entire disk image. Grow-on-demand images are always going to be slow
because every time the host has to grow the image, it hits the journal on
the host filesystem.

Dan.
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