linux-ext4 - Re: Introducing Next3 - built-in snapshots support for Ext3

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Message-ID: <AANLkTinbzARSNHOvMbRq7iKSZENKhnKYdKSq5IHvX_xs@mail.gmail.com>
Date:	Sat, 8 May 2010 07:43:22 +0200
From:	"Amir G." <amir73il@...rs.sourceforge.net>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Andi Kleen <andi@...obates.de>, tytso@....edu,
	linux-ext4@...r.kernel.org
Subject: Re: Introducing Next3 - built-in snapshots support for Ext3

On Fri, May 7, 2010 at 11:25 PM, Ric Wheeler wrote:
> On 05/07/2010 03:22 PM, Amir G. wrote:
>>
>> In theory, it is possible to have 2 modes for Ext4 (extents or snapshots)
>> and some would argue that it makes sense to do that.
>> But I think that making that decision can be deferred to a later time,
>> after people have experienced with Next3 and have decided if they
>> would like to have
>> the snapshot feature merged into Ext4 or not.
>>
>> Besides, it would take me a considerable amount of time to merge the
>> snapshot feature into Ext4,
>> and Next3 is ready to be used now.
>>
>> Amir.
>> --
>>
>
> I think that the counter argument would be that moving features into ext3 is
> probably the wrong thing to do.
>
> I don't think that anyone is in a huge hurry given that we have LVM based
> snapshots with ext3 and btrfs snapshots around the corner.  Probably this is
> most interesting when done to the latest version of the ext family.
>

This is a valid argument, but it is important for me to clarify a few
issues regarding the statements above:

1. No features are added to Ext3, so there is no concern for the
stability of Ext3.
The feature is added as a new f/s, with the slight overhead of
duplicate code in the
kernel tree and an extra loadable module in the system.

2. From the user's point of view, there is not much difference between
"mount -t next3"
and "mount -t ext4 -o snapshots", because in both cases it would not
be possible to
mount ext4 with extents support on that volume before discarding snapshots and
it will be possible to mount ext4 with extents support after
discarding snapshots.

3. Next3 snapshots are much more scalable durable and efficient than
LVM snapshots.
These are some of the benefits of built-in snapshots support.

4. I do not want to restart the discussion about when btrfs will be
production ready.
As for Next3 stability, I think that with the help of the community,
Next3 can be production ready within a matter of months,
because the Next3 code religiously attempts to retain the stability of
its ancestor Ext3.

I dare you to prove me wrong ;-)

Amir.
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