Re: [Xen-users] xen storage options - plase advise

["Matthew Law" <matt@xxxxxxxxxxxxxxxxxx>]

On Thu, March 4, 2010 12:38 am, James Pifer wrote:
>> LVM is _disk-based_ and not usable as filebacked-Store for XEN, but its
>> smarter to use LVM ;-)
>
> Thomas,
>
> Thanks for all the information so far, but of course I have more
> questions. I just want to make sure I understand it.
>
> Let me give you an example. Let's say I have five Windows 2008 servers.
> Right now they are using file based, growable storage.
>
> They each have a disk0 that is growable and partitioned like:
> c: = 25gb
> e: = 175gb (using round numbers)
>
> I understand the danger of using growable storage and having your
> storage fill up, but instead of these five machines using 200gb each, or
> one TB, they are currently only using 10-20gb each so far. (the sizes
> were spec'ed by the project, not my choice).
>
> Anyway, how would these work in LVM? Would I have to actually allocate
> 200gb for each of these on the physical disk?
>
>
> I also have linux pv domUs that have 40 or 50gb disks. They don't need
> to be that big, but is the way they were created. I currently have three
> dozen domUs which are a mix of linux (sles) and windows. Having to
> actually allocate the disk space that they are virtually using now would
> be a lot.
>
>
> Thanks for the example scripts. I would really like to be able to do
> something like that.

James,

I had similar questions a while ago and back then the consensus was that
snapshotting a running domU does *not* provide a consistent backup.  This
is especially the case with databases and applications which do not
immediately flush writes to disk.  I believe XenServer may do a better
job, but I am unsure on this.

Basically, the only way to do a proper backup is to properly shutdown the
domU before taking the snapshot, thus ensuring there is no chance of
losing data.  For some people not doing this is an acceptable risk and
certainly better than no backup at all.

With regard to storage, the industry buzzword for what you describe is
'thin provisioning'.  This, together with the de-duplication features
offered by some SANs makes for very efficient use of storage space.  If I
do this with Solaris for example, it shouldn't really matter if I use a
file system or LVM on the exported volume, because all I am doing is
telling ZFS to export a zvol and the dom0 sees this as block-level
storage.  I might set the maximum size of the volume to much more than the
space currently available and as it fills up I just need to ensure I keep
pace and add more physical storage as needed.  Your SAN product may well
allow you to do the same thing.  The advantage of LVM is that you can
later add further space with minimal hassle.

If you need to use CLVM and lose LVM snapshots, could you take snapshots
directly from your SAN instead?

Hope this helps,

Matt.



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