Re: [Qemu-devel] [RFC] qemu snapshot enchancement

[Dietmar Maurer]

> > Are you sure this work on shared iSCSI devices (I have my doubts)?
> 
> If by "shared" you mean clustering support so multiple hosts can access
> volumes from the same pool, then the answer is no.

Unfortunately this is the standard setup with our environment.

> If by "shared" you mean that there are iSCSI devices which are attached to a
> single host at a time but the mapping can change, then that should work fine.
> 
> > Besides, we use RHEL6.3 kernel, and AFAIK dm-thin is still not marked
> stable there.
> 
> Yeah, it's still a fairly new feature.  The key feature is that it uses a 
> single
> storage pool which all snapshots share instead of the insane per-snapshot
> volumes that need to be managed for classic LVM snapshots.
> > > Or is LVM a pain for other reasons?
> >
> > a.) it only works on LVM (new solution works on any storage type)
> > b.) You need free space on the VG (almost every user configures LVM
> > without leaving any free space on the VG).
> > c.) you need to specify snapshot size in advance
> > d.) performance is bad on writes.
> >
> > The new solution work perfectly on all storage types, works on shared
> > storage, and does not need any free space on the source storage.
> 
> These are good points and it sounds like QEMU-level snapshots work great
> for you.
> 
> QEMU should support both our own snapshots as well as exploiting features
> in lower levels of the storage stack.  That way users can choose the best
> option for their use case.

Sure.