linux-ext4 - Re: Question about writable ext4-snapshot

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Date:	Mon, 23 Jan 2012 22:08:49 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Robin Dong <hao.bigrat@...il.com>, Tao Ma <taoma.tm@...il.com>,
	coly <colyli@...il.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Yongqiang Yang <xiaoqiangnk@...il.com>
Subject: Re: Question about writable ext4-snapshot

On Mon, Jan 23, 2012 at 5:21 AM, Ted Ts'o <tytso@....edu> wrote:
> On Sun, Jan 22, 2012 at 11:31:31AM +0800, Robin Dong wrote:
>> > At the end of the day, thinp target is a very powerful tool, but
>> > is does not fit all use cases. In particular, it fragments the
>> > on-disk layout of ext4 metadata and benchmark results for how this
>> > affect performance were never published.
>
> Amir,
>
> Well, to be fair, your approach to snapshotting also causes
> fragmentation.  If a file or a directory in the base image gets
> modified while there is a read-only snapshot, the inode in the base
> image gets fragmented as a result.

Yes, that's true, to some extent. directory inodes, however, do not
get fragmented. all journaled metadata is copied a side on JBD hooks.
My claim was about fragmentation of ext4 metadata, but fragmentation
of data is also a problem in both approaches.

>
> It is true that thin provisioning in general tends to defeat the block
> placement algorithms used by a file system, but it will be possible to
> create snapshots of non-thinp volumes, which will address this issue.
> Hopefully in the next 3-6 months, these things will be implemented
> enough so that we can benchmark them and see for certain how well or
> poorly this approach will work out.  I'm sure there will be a certain
> number of tradeoffs for both approaches.
>
> Regards,
>
>                                        - Ted
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