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Re: [Xen-users] Questions on qcow, qcow2 versus LVM

On Wed, Dec 23, 2009 at 8:31 PM, Matthew Law <matt@xxxxxxxxxxxxxxxxxx> wrote:
> Now I am considering two options:
> 1) If this issue is confined to LVs used for swap disks, perhaps I can
> switch to using disk files for domU swap space..?

That ... depends.
Generally, performance-wise files will not be as good as block device
(partition, LVM, etc.) That being said, if you correctly predict domUs
resource assignement so that swapping rarely occures (it kills
performance anyway), and it's just a safety net against OOM, you
probably won't notice the performance difference.

> 2) If this isn't confined to LVs used for swap, then perhaps I could have
> an opensolaris domU export zvols across NFS or iSCSI back to the dom0 and
> use these for each domU system and swap disk? - this sounds a little crazy
> to me and performance and load might be unacceptable too.

That would work. Sun even sells iscsi SAN server based on zfs, called
Sun Unified Storage Systems.
Note however that in my test, even on the same server, zfs + zvol
performance is lower compared to LVM. Add to that iscsi and network
overhead. Whether or not it's acceptable depends on your requirement,
so it's best to try it yourself.

> Are there any other options available?

If you don't care about space saving (I seem to recall you mentioned
snapshot in another thread), you can just simply use the "disk"
directly on domU as swap. That is, you assign two disks to domU, one
of them for filesystem, the other as swap. Don't label the swap disk,
don't create partitions, just use mkswap directly. In my case I assign
it directly as partition (h/s/xvda1, xvda2, etc) but you can assign it
as disk so it would work better with GUI tools

Another thing to note if you use LVM snapshot, if somehow you let the
snapshot fill to 100%, you might lose data. That's why I only use
snapshots for temporary purposes. It might not be a problem if you can
guarantee that it will always be below 100% (perhaps with some
monitoring/alert system), but IMHO it's not worth it.


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