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

[Joe Thornber]

On Sat, Jun 11, 2011 at 08:41:38AM +0300, Amir G. wrote:
> On Fri, Jun 10, 2011 at 1:11 PM, Joe Thornber <thornber@xxxxxxxxxx> wrote:
> > On Fri, Jun 10, 2011 at 11:01:41AM +0200, Lukas Czerner wrote:
> >> On Fri, 10 Jun 2011, Amir G. wrote:
> >>
> >> > CC'ing lvm-devel and fsdevel
> >> >
> >> >
> >> > On Wed, Jun 8, 2011 at 9:26 PM, Amir G. <amir73il@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >> > For the sake of letting everyone understand the differences and trade
> >> > offs between
> >> > LVM and ext4 snapshots, so ext4 snapshots can get a fair trial, I need
> >> > to ask you
> >> > some questions about the implementation, which I could not figure out by myself
> >> > from reading the documents.
> >
> > First up let me say that I'm not intending to support writeable
> > _external_ origins with multisnap.  This will come as a suprise to
> > many people, but I don't think we can resolve the dual requirements to
> > efficiently update many, many snapshots when a write occurs _and_ make
> > those snapshots quick to delete (when you're encouraging people to
> > take lots of snapshots performance of delete becomes a real issue).
> >
> 
> If I understand this article correctly:
> http://people.redhat.com/mpatocka/papers/shared-snapshots.pdf
> It says that _external_ origin write updates can be efficient to readonly
> (or not written) snapshots.
> 
> Could you not support readonly snapshots of an _external_ origin?

Yes, that is the intention, and very little work to add.  We just do
something different if the metadata lookup returns -ENODATA.  Above I
said I didn't intend to support _writeable_ external snaps.  Readable
ones are a must, for instance for supporting virtual machine base
images.

> You could even support writable snapshots, that will degrade write
> performance to origin temporarily.
> It can be useful, if one wants to "try-out" mounting a temporary
> writable snapshot, when the origin is not even mounted.
> After the "try-out", the temporary snapshot can be deleted
> and origin write performance would go back to normal.

Not sure what you're getting at here.  All snapshots are writeable.

Of course you can take a snapshot of an external origin and then use
this as your temporary origin for experiments.  If the origin is
itself a dm device then LVM can shuffle tables around to make this
transparent.

The user may want to commit to their experiment at a later time by
merging back to the external origin.  This involves copying, but no
more than a copy-on-write scheme.  Arguably it's better to do the copy
only once we know they want to commit to it.

- Joe
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