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

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Message-ID: <BANLkTi=0N7QXRvFgjHUJXBi0x=668ahXXA@mail.gmail.com>
Date:	Sat, 11 Jun 2011 08:41:38 +0300
From:	"Amir G." <amir73il@...rs.sourceforge.net>
To:	Joe Thornber <thornber@...hat.com>
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 Fri, Jun 10, 2011 at 1:11 PM, Joe Thornber <thornber@...hat.com> 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@...rs.sourceforge.net> 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?

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.

Is that correct?

Amir.
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