userspace tool to freeze/thaw ext3 to create consistent snapshots

[Andreas Dilger]

On Sep 13, 2007  00:53 +0200, Filip Sneppe wrote:
> Suppose one has an ext3 filesystem on a SAN LUN.
> I would like to know if it is possible to freeze this ext3 filesystem
> into something
> consistent from a filesystem point of view, then trigger the SAN's snapshotting
> functionality, and then thaw the ext3 filesystem to resume I/O. This
> would allow
> near-instantaneous snapshot backups without having to use LVM or
> LVM snapshots.

You could probably make an ioctl to ext3 to do this, calling the existing
code to flush the journal during snapshots.

> So, my questions are:
> - Is this currently possible from userspace with ext3 ? If not, is this
>   hard to write ? (I am not an expert programmer, but from looking at the
>   XFS and GFS userspace code, things are done totally differently)
> - Given that all Linux FS support suspending of I/O operations,
>   is it technically possible to write a generic userspace tool to do just
>   that: freezing/thawing I/O requests to a mountpoint from userspace,
>   no matter what the underlying FS is ?

Yes, if there is a common ioctl or syscall - the filesystems themselves
have a common method to do the freeze/thaw.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.