linux-ext4 - Re: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots)

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Message-ID: <20110613085054.GA4580@ubuntu>
Date:	Mon, 13 Jun 2011 09:50:55 +0100
From:	Joe Thornber <thornber@...hat.com>
To:	"Amir G." <amir73il@...rs.sourceforge.net>
Cc:	Lukas Czerner <lczerner@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu, linux-kernel@...r.kernel.org, lvm-devel@...hat.com,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots)

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
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