linux-ext4 - Re: Next3 on-disk format change request

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Message-ID: <AANLkTinGt6quy39d2urgkkZzE0igkjFvy7fdRstcrQeJ@mail.gmail.com>
Date:	Tue, 21 Dec 2010 14:28:01 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	tytso@....edu
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: Next3 on-disk format change request

On Thu, Jun 24, 2010 at 7:33 AM, Amir Goldstein <amir73il@...il.com> wrote:
> On Thu, Jun 24, 2010 at 3:35 AM,  <tytso@....edu> wrote:
>> On Tue, Jun 22, 2010 at 01:30:54PM +0300, Amir Goldstein wrote:
>>> My on-disk format change request is to re-assign a ro_compat feature
>>> for IS_SNAPSHOT (currently in s_flags).
>>
>> OK, so in addition to:
>>
>> #define EXT4_FEATURE_RO_COMPAT_HAS_SNAPSHOT     0x0080
>>
>> You would also like:
>>
>> #define EXT4_FEATURE_RO_COMPAT_IS_SNAPSHOT      0x0100
>>
>> which is a feature flag that the kernel will never support, since this
>> is a hint that snapshot should only be mounted read-only, correct?
>>
>
> That is correct.
>

Hi Ted,

I am pinging you on this old request of mine... and modifying my
request slightly (to an incompatible feature).

I would like to have EXT4_FEATURE_INCOMPAT_IS_SNAPSHOT,
which will be set only inside snapshot images.

This is to say that Ext4 snapshots can only be mounted by Ext4 with
'snapshot mount' support.

One of the important applications of 'snapshot mount' incompatible
support is to deny read access
to snapshot files (or more generally excluded files) entires inside a snapshot.

This issue is currently handled by clearing all old snapshot inode
copies on snapshot take time.
It is a sub optimal solution, which was imposed by the design to mount
Next3 snapshot as Ext2
and it is not scalable to a large number of 'excluded' files.

Thanks,
Amir.
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