On Wed, 30 Sep 1998 14:08:27 -0400 (EDT), "Adam D. Bradley"
<artdodge@cs.bu.edu> said:
> Actually, "snapshot" functionality is remarkably easy to do with an LFS,
> and I imagine it's not that difficult for a JFS either. Both have the
> advantage that old data can be pushed off to tape or other slow media once
> it's no longer "live", but supporting this well would probably require
> userspace help.
Snapshots are almost trivially easy with LFS. On the other hand,
having a journaled filesystem is absolutely no help at all for
constructing snapshots: journals are short-term activity logs, and
snapshots need long term records of multiple versions of the
filesystem.
Journaling does not help with migrating data to tape, either. Those
are completely separate issues.
Right now, the only Linux filesystem that could be modified to support
snapshots without a complete redesign is the tree-based Reiserfs: if
you can support multiple root blocks representing distinct but
overlapping filesystems, then you have everything necessary for
snapshots.
One thing that we CAN conceivably do with ext2fs is to implement
multiple versions for files (the redesign of directories for btree
support will make this relatively simple). That is very, very
different from snapshots, though. Supporting data migration to tape
is a completely different issue, and we can do that without using any
logging or journaling features.
--Stephen
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