[linux-lvm] Snapshots

[Mike Snitzer]

On Thu, Feb 16 2012 at 10:18am -0500,
Stuart D. Gathman <stuart at bmsi.com> wrote:

> Long ago, Nostradamus foresaw that on Feb 16, Mark Woodward would write:
> 
> >I guess I missed that. I need to check out the changes, looks like
> >you have auto-grow with monitoring? I was looking more towards the
> >"snapshots of snapshots" ability. Is that on the radar?
> 
> The ZumaStor 3rd party userland tools support that.

Now AFAIK ZumaStor is a dead technology... nobody is actually
maintaining the code upstream.  The shared snapshot store that
dm-thin-pool provides is functionally equivalent to what Zumastor
provides -- dm-thinp provides a single storage pool for all snapshots.
This avoids excessive copy-out that the old dm-snapshot suffered from
due to it requiring a distinct snapshot store for each snapshot.

I'd really like to encourage you and others to evaluate and test
snapshots with dm-thinp -- now that lvm2 provides support it _should_ be
more approachable.

> An elegant part of the LVM system is that the device mapper kernel support is
> very general, and new data structures can be experimented with entirely in user
> code - with a script language even.  Metadata for experimental structures does
> not have to stored with the main metadata.

Please note that the dm-thinp code has metadata in the kernel (on-disk
format for btrees, etc) much like a filesystem would have.  So there is
both kernel and userspace (lvm2) metadata for dm-thinp.

Mike