Re: [Xen-users] Re: Re: Redundant server setup

["Paul M." <paul@xxxxxxxxxx>]

On 5/23/06, Christopher G. Stach II <cgs@xxxxxxxxx> wrote:
> Paul M. wrote:
> > If you have a few machines to devote to file storage than a cluster
> > file system is the best way to go. Having dom0 on each of the machines
> > provide a CFS<-->NFS bridge for the domUs and a Nagios server running
> > to monitor services/domU's and restart on other machines if needed
> > would provide a system with the network being the only single point of
> > failure. And that could be taken care of too....
> > -Paul
>
> I would only recommend involving NFS for those users who have been
> delegated the responsibility of throwing performance into the toilet.
> There's no need to involve an extra lock manager and incur extra
> communications overhead, and more processing for dom0.  It could even be
> dangerous.
 The NFS would be extra overhead but it would allow the DFS to work
on a per file basis instead of with an image of the hard disk.
Performance and irreparable corruption are possible problems with the
disk image.
 Some DFSs might have problems dealing with the frequent, minor
changes to disk image. There may also be problems if the disk image is
larger than the local cache. Much of this would depend on which DFS
you went with.
 With the DFS working on a per-file basis corruption would be limited
to application level corruption of a few files. With a disk image
related writes (ex updating an inode and adding some data) may not get
placed in the same transaction (assuming your DFS worked like this).
Thus the chance of data loss or total failure is higher than if
working on a per file basis.
 Of course if there is a way to use a local directory as a domU root
then you can avoid the problem completely.
-Paul

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