Re: xt4 - True Readonly mount [WAS - Re: [Bug 14354] Bad corruption with 2.6.32-rc1 and upwards]

[Greg Freemyer <greg.freemyer@gmail.com>]

On Sun, Nov 1, 2009 at 12:45 AM, Theodore Tso <tytso@mit.edu> wrote:
> On Fri, Oct 30, 2009 at 10:20:35AM -0400, Greg Freemyer wrote:
>> Ignoring computer forensics, with LVM snapshots, hardware raid array
>> snapshots, etc. even in the presence of a dirty log, we need to be
>> able to mount a drive in true read-only fashion fro many backup
>> operations to function correctly.
>
> Can you go into more detail about "many backup operations"?

One example is a hardware raid array that creates readonly snapshots
or clones.  (Lots of those exist in the real world).

So the typical backup procedure is:

====
Queisce application (databases, etc. have utils to do this.)

Queisce filesystem (xfs_freeze -f can be done from userspace. is there
a ext4 util?)

issue raid array command to create snapshot.

release filesystem (xfs_freeze -u)

release the app (util provided by app).

Mount the snapshot readonly (true readonly with zero writes to the
block device).

Backup the readonly snapshot (to tape, etc.).
===

I believe XFS had 2 issues related to this process when first
implemented in linux.

1) It required the UUID to be unique.  Obviously in the above scenario
it is not, so "mount -o nouuid" was added for xfs.

2) Journal replay was originally aways attempted in the above process,
so the "mount -o norecovery" option was added to force a true readonly
mount.

ext4 may already support mounting of readonly clones, but if not it
needs to before it will qualify as a data center ready filesystem.

Greg
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Greg Freemyer
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