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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


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