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

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Message-ID: <BANLkTi=4=YkZezxkFh80JAwjH16Fu8PFXA@mail.gmail.com>
Date:	Thu, 9 Jun 2011 10:05:42 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Sunil Mushran <sunil.mushran@...cle.com>
Cc:	"Amir G." <amir73il@...rs.sourceforge.net>,
	Mike Snitzer <snitzer@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, tytso@....edu,
	linux-kernel@...r.kernel.org, sandeen@...hat.com
Subject: Re: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots)

On Thu, Jun 9, 2011 at 2:49 AM, Sunil Mushran <sunil.mushran@...cle.com> wrote:
> On 06/08/2011 11:26 AM, Amir G. wrote:
>>
>> 2. Data blocks are never copied
>> The move-on-write technique is used to re-allocate data blocks on rewrite
>> instead of copying them.
>> This is not something that can be done when the snapshot is stored on
>> external storage, but it can done when the snapshot file lives in the fs.
>
> But does that not lead to fragmentation. And if I am understanding this,
> the fragmentation will not resolve after dropping the snapshot. So while
> you do save the overhead on write, you make the user pay on all future
> reads (that need to hit the disk).
Hi Sunil,

I am undertaking a project which aims to reduce fragmentation when the
file is rewritten.  When the snapshot is deleted, fragmentation
introduced by snapshot will be removed once the file is rewritten.  If
it is necessary, I think defragmentation can be done when the file is
read.

But it is not ready yet.  So it is not a part of the code which are
going to be merged.

Does I answer your question?
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-- 
Best Wishes
Yongqiang Yang
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