Re: [Xen-users] Newbie Question on Comparisons/Advice

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I've read (have not done comparisons for myself) that a loopback-mounted
> file in a Linux system will give comparable performance to a raw device
> (i.e. LVM volume).  The advantage of LVM is in the flexibility, but
> you're in a situation where the up-front costs (shrinking filesystem,
> spending some time with the LVM docs) might outweigh the advantages.

LVM is generally recommended for Xen guest storage for reasons of performance, 
flexibility and robustness.  Basically, it's about as fast as using raw disk 
but with the flexibility of being able to easily resize, snapshot, etc.  
Using files was sort-of not recommended for production environments since the 
Linux loopback device has some limitations (performance, scalability and 
flexibility, I think)

I've found that loopback devices are perfectly adequate for playing around 
with stuff on a small system, however.  Also, using tap:aio: instead of file: 
allows you to make a regular Linux file available as a VBD without the 
problems of the block loopback.  I think that's the recommended way to do 
things these days.

> If you want to do it that way, yeah, you'll need to shrink the
> filesystem, then you'll need to shrink the (virtual) device it lives on,
> and then you can create a new virtual device in the freed space.  But
> it's quicker just to dd a bunch of zeroes into a file and treat that as
> a disk.

Indeed!

> > This now brings me back to having to start over reinstalling Fedora;
> > from my understanding.
>
> Well, not necessarily, unless the filesystem shrinking damages your
> installation, but given the possibility of that I'd go for a loopback
> file if it were me.

To be honest, if you have a Qemu hard disk image lying around from your 
previous experiments you *might* be able to simply point Xen at it using 
tap:aio: and have it boot.  Windows would probably cope with detecting the 
various changes in virtual hardware.  The major caveat here is that 
(depending on your Windows version / license) Windows can get difficult and 
decide that it's been moved to another machine and wants to be reactivated.  
If you've got limited connectivity or a limited number of allowed 
activations, you'll not want to do this.

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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