Options for synchronising filesystems
olli at lurza.secnetix.de
Tue Sep 27 04:25:29 PDT 2005
Isaac Levy <ike at lesmuug.org> wrote:
> In effect, a terse example of how someone could use the Geom tools I
> mentioned, to meet this requirement:
> + Setup mirrored disks across machines as discussed before
> + Mount a slice of that disk Read/Write on one machine (acting as
> + Mount that same slice Readonly on both machines, using Geom Gate,
> and serve data from there.
That doesn't work. You would need a cache coherency proto-
coll for that setup to work correctly. Or mount it read-
only _everywhere_ (including the master). If you have to
perform updates, you would have to perform this sequence:
1. remount the master read-write,
2. do the updates,
3. remount the master read-only,
4. flush the caches on the slaves (umount; mount).
But that means you'll have a short downtime each time you
update things -- probably not what you want, especially if
a redundant setup is the goal.
Currently, the _only_ way to mount the same filesystem on
multiple FreeBSD systems is NFS (or third-party software
like CODA). Another alternative, as others have mentioned,
is to duplicate or synchronize the filesystems on each
server regularly (rsync, unison, whatever).
> Eric, you are definately correct, that there's not really a disk-
> level mechanism to maintain concurrent writes between volumes mounted
> across servers using FreeBSD
Not even concurrent reads and write (i.e. one host writes
and the others read). See above.
> (excepting NFS, which in this context, makes me say *yuck*).
NFS certainly has its disadvantages, but works pretty well
when set up in a reasonable way.
> I guess I'm saying this is a big-picture computing problem IMHO, and
> I don't know of a good solution here (though I'm curious about what
> kind of work has been done in Dragonfly which is relevant?)
So far, DragonFly BSD hasn't done anything regarding cache-
coherency for clusters, though it is planned for the future,
I think. So, as of today, DF doesn't provide a solution for
the above problem either.
However, Matt Dillon has worked on a journalling feature
for UFS which might be helpful for the situation given by
the OP. Not everything is implemented yet, but it _is_
already usable to maintain remote mirrors. On July 5th,
Matt wrote: "I can now run a buildworld loop, and the
mirror generated by the journal stays in synch (diff -r
reports no differences if I idle the buildworld, wait a
second or two for the journal to catch up, and run it)."
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
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