[CentOS] Query: Filesystems
maze at cela.pl
Fri Nov 25 00:27:14 UTC 2005
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
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...
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