Hello, Just a few quick, but not very simple questions...: 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. Can this (freezing) be used for the root filesystem or other important filesystems? ie. might it cause something vitally important to freeze? 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). And the last question: is there some sort of filesystem which supports generating 'in-filesystem' snapshots (ie. some sort of copy-on-write block semantics applicable both to files and directories combined with both a root rw directory to access current files and a ro /snapshot/YYMMDD-HHMMSS hierarchy to access snapshots from past dates with the possibility of deleting them at will (ie. generate a snapshot every hour, after 24 hours leave only every 6th one, after 7 days leave only dailies, after a month leave only weeklies etc... - much too complicated, but it's easier to get the basic idea across on a larger example - probably organized via cronjob). I realise such a solution would not be foolproof (if the disk goes bad or the filesystem goes haywire it's pretty much useless) so a hard-copy would still need to be maintained (probably via freeze/snapshot/unfreeze/dump it somewhere else) - but this would be useful for the users (the ability to restore past deleted files...) In a way this could possibly be realized via some twidling with rsync (although for space efficiency this would require copy-on-write hardlinks or something like that) but I see no way of doing this without wasting a lot of space (at least 1 copy of the snapshoted filesystem, the rest could probably be hardlinked). And to maintain full R/O status, this would need to be a seperate partition, etc... Any comments/ideas? Cheers, MaZe.
Maciej ?enczykowski wrote:> And the last question: is there some sort of filesystem which supports > generating 'in-filesystem' snapshots (ie. some sort of copy-on-write > block semantics applicable both to files and directories combined with > both a root rw directory to access current files and a ro > /snapshot/YYMMDD-HHMMSS hierarchy to access snapshots from past dates > with the possibility of deleting them at will (ie. generate a snapshot > every hour, after 24 hours leave only every 6th one, after 7 days leave > only dailies, after a month leave only weeklies etc... - much too > complicated, but it's easier to get the basic idea across on a larger > example - probably organized via cronjob).WAFL does this, unfortunatly it only runs on NetApps. -jim
On 11/24/05, Maciej ?enczykowski <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' snapshotsIt 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.
On Fri, Nov 25, 2005 at 01:27:14AM +0100, Maciej ?enczykowski wrote:> Hello, > > Just a few quick, but not very simple questions...: > > 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 danglingNot a linux filesystem, but sun's new ZFS filesystem has great snapshotting support, as well as a million other incredible features. It's more than what we think of as a traditional filesystem - it incorporates software raid & volume management functionality as well. danno