[Fedora-xen] Best practices questions

[Daniel P. Berrange]

On Mon, Dec 03, 2007 at 10:48:59AM +0100, Markus Armbruster wrote:
> "Lopez, Denise" <dlopez at humnet.ucla.edu> writes:
> 
> > Hi all,
> >
> >  
> >
> > I am in the process of building a new Xen server from scratch and wanted
> > to ask a couple of questions about best practices.  
> >
> >  
> >
> > First, should the guest domains be image files or LVM's or just regular
> > ext3 partitions? What are the pros and/or cons of each?
> 
> Image files are slow.  Sparse image files lead to *nasty* domU data
> corruption when you run out of space in the filesystem containing the
> image file.

Actually that's not true. You will definitely get I/O errors in the
DomU if Dom0 runs out of space. Journalling FS will save you though.
If you kill the guest, add more Dom0 space, and start the guest again
it should recover just fine. This is why I went to great lengths to
fixing Xen 3.0.3 to make sure I/O errors are actually propagated to 
the guest, rather than being dropped on the floor in Dom0 pretending
it was all OK like in the bad Xen 3.0.2 days.

The issue with sparse files is that they have really bad performance
characteristics whe nyou do writes, becaue each time Dom0 as to allocate
an extra block behind the sparse file it hits the journal, and this 
basically serializes all I/O requests in the guest. Once the sparse
file is fully-allocated, it should be good again.

> LVM vs. regular partitions is the usual deal: LVM gives you more
> flexibility.  You might find that you appreciate the flexibility even
> more when you run guests.

LVM & partitions give best performance characteristics.

Regards,
Dan.
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