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

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Date:	Wed, 30 Mar 2011 14:08:50 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Joel Becker <jlbec@...lplan.org>,
	lsf-pc <lsf-pc@...ts.linuxfoundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Theodore Tso <tytso@....edu>, Josef Bacik <josef@...hat.com>
Subject: Re: [LSF/FS TOPIC] Ext4 snapshots status update

On Wed, Mar 30, 2011 at 1:50 PM, Chris Mason <chris.mason@...cle.com> wrote:
> Excerpts from Amir Goldstein's message of 2011-03-30 00:16:45 -0400:
>> On Wed, Mar 30, 2011 at 2:34 AM, Joel Becker <jlbec@...lplan.org> wrote:
>> > On Wed, Mar 23, 2011 at 10:19:38PM +0200, Amir Goldstein wrote:
>> >> 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'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.
>> >>
>> >> Any chance of getting a copy of that design of yours, to get a head start
>> >> for LSF?
>> >
>> >        Yeah, I owe it to you.  It wasn't a written-down thing, it was a
>> > hammered-out-in-our-heads thing among some ocfs2 developers.  I'm going
>> > to braindump here to get us going.  First, I'll speak to your points.
>> >
>> >> 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.
>> >
>> >        I'm unsure where these fit, perhaps because I missed the
>> > discussion between Chris and you.  ocfs2 has the inode flag
>> > OCFS2_REFCOUNTED_FL to signify a refcount tree is attached to the inode.
>> > This is ocfs2's structure for maintaining extent reference counts.  Is
>> > your COW_FL the same?  Or is it a permission flag?  NOCOW_FL sounds
>> > like: "Set this flag on the inode and it will prevent CoW."
>>
>> I don't have a use for COW_FL, since my snapshots are volume level snapshots.
>> I intend to use NOCOW_FL to mark an inode as an "island" of NOCOW
>> blocks in the volume.
>> Maybe Chris or Josef can elaborate of the flags intended use in btrfs.
>
> NOWCOW_FL in btrfs means to directly overwrite blocks (and not do crcs)
> unless the block has another reference.  If there is another reference,
> we COW once to honor the snapshot and then continue in NOCOW mode.
>
> I'm kind of worried about your NOCOW island idea, maybe we can talk more
> about that next week.  It seems like it will lead to a lot of admin
> surprises.
>

Yes, that's something to talk about.
My desire for NOCOW comes from lack of sub volume granularity
in ext4 snapshots.

My NOCOW design states that NOCOW flag cannot be toggled on a regular file.
like a snapshot file, a NOCOW file must be born and die NOCOW, to avoid
admin surprises. NOCOW directories (which ARE COWed) are were NOCOW
files are born.

Using this scheme, an admin can exclude->include->exclude directory sub trees
from snapshots.

Amir.
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