Ext3 users - Sep 2007 - userspace tool to freeze/thaw ext3 to create consistent snapshots

Hi everyone,

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.
>From googling around, I understand that:
- LVM can take consistent snapshots at the FS level, and from posts
  to LKML, I understand that all Linux filesystems now support suspending I/O
  activity during these snapshots as a requirement for LVM snapshotting.
- XFS has a userspace command "xfs_freeze [-u] mountpoint" which
  appears to do exactly what I want to achieve with ext3
- GFS has a userspace command "gfs_tool [un]freeze mountpoint" which
  appears to do the same thing

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 ?
>From the various posts with similar questions that pop up on
various mailing lists and support forums, it would appear to me that
there is some level of interest/demand for this type of feature.

Regards,
Filip

Filip Sneppe wrote:
> Hi everyone,
>
> 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.
>   
I think (but I might be wrong) that this feature is not implemented -
yet - for ext3fs.
However, LVM2 should allow to create read-write snapshots.
Therefore, you can snapshot whenever you want, then mount the snapshot,
like you would mount an ? unclean ? filesystem (i.e. the log will be
replayed, and that's it).
One might argue that this could give unconsistent snapshots ; however
(the gurus will tell if I'm wrong) since your programs are running when
you do the snapshot, it should not change anything whether you do a ?
hot ? snapshot or a ? thawed ? snapshot.

regards

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.