linux-ext4 - Re: [LSF/FS TOPIC] Ext4 snapshots status update

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Date:	Wed, 23 Mar 2011 22:19:38 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Joel Becker <jlbec@...lplan.org>
Cc:	lsf-pc@...ts.linuxfoundation.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Theodore Tso <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [LSF/FS TOPIC] Ext4 snapshots status update

On Fri, Feb 4, 2011 at 2:20 AM, Joel Becker <jlbec@...lplan.org> wrote:
> On Fri, Feb 04, 2011 at 12:33:39AM +0200, Amir Goldstein wrote:
>> I have been working on porting Next3 snapshots to Ext4
>> with a group of 4 CS students.
>>
>> This is where it all happens:
>> https://github.com/amir73il/ext4-snapshots
>>
>> Ext4 snapshots prototype is already working!
>> Would you like to see a demo?
>>
>> I would like to present the progress of our work
>> and discuss the remaining issues and how they should be best addressed.
>>
>> Additionally, I would like to discuss the need for a unified snapshots API that
>> would serve both Ext4 and Btrfs.
>> This could be useful for someone that wants to implement a generic
>> snapshots management system.
>
>        ocfs2 definitely wants to be a part of that discussion, as we
> already do snapshots and thin clones.  ocfs2 snapshots are actually just
> thin clones marked readonly.  They can be placed anywhere in the
> filesystem and are first-class inodes.  They do not live in a hidden
> space like Next3 snapshots seem to.
>        I've already got a design for a front-end snapshot program that
> implements a policy on top this generic behavior.  This design would
> cover both first-class and hidden style snapshots, because it assume
> snapshots are in a distinct namespace.  I haven't gotten around to
> implementing it yet, but btrfs and other snapshottable filesystems were
> part of the design goal.
>
> Joel
>

Hi Joel,

Any chance of getting a copy of that design of yours, to get a head start
for LSF?

Here are some other generic snapshot related topics we may want to discuss:

1. Collaborating the use of inode flags COW_FL, NOCOW_FL, suggested by Chris.
2. How to deal with mmap write to COW file, when you get ENOSPC.
3. Adding buffer_remap() flag for buffered I/O code, meaning, there is
an existing mapping to initialize a page on partial write, but still need
to call get_block() to get a (possibly) new mapping.

I'll be staying for Collaboration summit, so I've got plenty of time
if you like to exchange some ideas.

Amir.
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