Re: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots)

[Joe Thornber <thornber@redhat.com>]

On Sat, Jun 11, 2011 at 12:58:26PM +0300, Amir G. wrote:
> I meant _readonly_ snapshots of a _writable_ _external_ origin,
> which is what ext4 snapshots provides.
> All snapshots are chained on a list that points to the origin and
> only the latest (active) snapshot metadata get updated on origin writes.
> When older snapshots lookup return -ENODATA, you go up the list
> to the newer snapshot and up to the origin.
> 
> Those _incremental_ snapshots cannot be _writable_, because older
> snapshots may implicitly share blocks with newer snapshots, but it should
> be possible to make _writable_ clones of these snapshots.
> Not sure what the implications are for deleting snapshots, because I am
> not familiar with all the implementation details of multisnap.

I deliberately ruled out chaining schemes like this because I want to
support large numbers of snapshots.  I believe Daniel Phillips
described a chaining scheme a while ago, and someone else implemented
it last year.  From a cursory glance through the code they posted on
dm-devel, it appeared to need a large in memory hash table to cache
all those chained lookups.

- Joe