Re: snapshot, checkpoints

[David Masover <ninja@slaphack.com>]

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Valdis.Kletnieks@vt.edu wrote:
| On Fri, 04 Jun 2004 13:10:05 +0200, Paul Wagland said:
|
|
|>Can't the same functionality be created with device mapper though? At
least
|>under linux anyway?

[...]

| 2) You also need a suitable write-barrier interlock to the filesystem, to
| basically force a flush-to-disk of all the incore data buffers, etc
(basically,
| you need to ensure that at the instant the snapshot is taken, the
on-disk copy
| is "clean" by fsck standards).


My main problem with this is that it seems to literally be a snapshot --
that is, a complete backup to another partition.  I think we should have
something like a version control system -- blocks (or files, or
clusters, or whatever granularity is best/easiest) which change after
the snapshot must be rewritten to somewhere else in the same partition.

This means that the original filesystem is recoverable (in case of
software trouble), and that the recovery takes very little time.  It
also means that things which don't change much don't get copies made.

This is even an advantage when you do want a separate partition/drive to
guard against disk failures, because you can now store many more
versions on a single partition.  I currently do something like this with
hardlinks -- every new backup is a hardlink farm of the latest backup,
which I then rsync with the live filesystem.  (rsync unlinks files
before it overwrites them.)  But that's hardlinks -- it'd be nice to be
able to do it at finer granularity (if there'd be no performance loss)
and on live partitions (without an actual "backup").

Of course, the necessity of a full snapshot is still there, and with
that, I'd rather have the files copied a la "cp" -- less disk activity.

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