On 2010-03-21, at 20:12, Matt Helsley wrote:
These are the same kinds of problems encountered during backup. You
can play fast and loose -- like taking a backup while everything is
running -- or you can play it conservative and freeze things.
I think btrfs snapshots are just one possible solution and it's not
overkill.
For some filesystems it might make sense to use the filesystem
freezer to
ensure that no files are deleted while the backup takes place.
Combined
with tools like rsync or rdiff backup these operations could be low
bandwidth
and low latency if well-known live-migration techniques are used.
Or use dm snapshots.
If you are using snapshots, then even an open-unlinked file will not
be deleted from the filesystem until it is closed, because the inode
will still be available on disk even without the filename. That would
be a good reason to also store the file handle (e.g. inode+generation
for simple filesystems) in the checkpoint file, so that you can re-
open this file by the file handle after the process is restarted.
Since Aneesh is starting to add an interface for this to the kernel
anyway, I don't think it would be very hard to dump/restore a handful
of extra bytes with each file. Conversely, now is the time for
getting the open-by-handle APIs correct for this code.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
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