[CentOS] Query: Filesystems

[Josh Kelley]

On 11/24/05, Maciej Żenczykowski <maze at cela.pl> wrote:
> Do any Linux filesystems (besides XFS) support freezing?
> (ie. in conjuction with LVM snapshots this can allow a mounted
> filesystem to be frozen [freezing all processes writing to this
> filesystem] in a valid state (with possibly dangling
> unlinked files), the device can be snapshotted via LVM, and the
> original filesystem unfrozen - the snapshot now contains a valid snapshot
> in time of the original filesystem - with no corruption.

I'm reasonably certain that LVM+ext3 automatically makes consistent
snapshots (no freezing necessary).  See
http://www.tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html for (slightly)
more details.

> Can a snapshotted LVM (the snapshot, not the original filesystem) be
> modified (I assume snapshots are purely readonly...) - so as to remove the
> dangling deleted/unlinked files and possibly modify the uuid (I realize
> there is a nouuid mount option for XFS).

Under LVM2, snapshots are read-write; however, LVM2 isn't currently
reliable on CentOS 4 or the upstream release.  (See the thread
starting at http://marc.theaimsgroup.com/?l=centos&m=113155162316509
for details.)

> And the last question: is there some sort of filesystem which supports
> generating 'in-filesystem' snapshots

It looks like there's a research project underway at
http://www.ext3cow.com/, and there's the Wayback filesystem for FUSE
at http://wayback.sourceforge.net/, but I've no idea how usable either
is.