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

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Message-ID: <20120123032143.GA16363@thunk.org>
Date:	Sun, 22 Jan 2012 22:21:43 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Robin Dong <hao.bigrat@...il.com>
Cc:	Amir Goldstein <amir73il@...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 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.

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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