On 13 Oct 2000, Bennett Todd <[EMAIL PROTECTED]> wrote:
> 2000-10-13-17:42:32 Mark W. Eichin:
> > Actually, systems like AFS (soon to be OpenAFS, in the next
> > month or so...) *do* "perfect" backups, involving creating
> > "backup volumes" (a useful thing to have around anyway,
> > basically a nearly-free clone) and then the offline backups are
> > made from those.
>
> How could I have forgotten! I didn't know about AFS's backup
> volumes, but I do know and value the analogous feature on NetApps'
> filesystem WAFL, namely "snapshots". As you say, it's nearly free.
My understanding is that a NetApp Filer might take a snapshot while a
file is in the middle of being written, so although rsync couldn't
crash, there is no guarantee that the file is consistent from the
application's point of view.
Generally speaking the information you need to do this 100% correctly
simply isn't there in Unix: unlike (say) Tandem, applications don't
tell the OS when the file is consistent and when it is not. One
option is to shut down the application (e.g. before remounting
read-only), which means it must be consistent. Another is for the
application to handle it using several approaches: fsyncs at the right
time, atomic-rename, append-only, and so on.
If we eventually get to Scriptable Rsync(TM), then the administrator
might be able to configure in things like locking mailboxes while
they're accessed. That's a while off, but if you're interested then
say so.
Another option sometimes discussed is that rsync should lock files as
it accesses them. It's an interesting approach, but there are a few
problems which have been thrashed out on this list previously. Again
I think the basic problem is that it would require too much
application and system-specific knowledge to belong in the
general-purpose rsync tool.
> I dunno if AFS's backup volumes achieve the same thing, but you can
> use WAFL's snapshots to keep online backups as well, which is really
> fun.
Search for the tux2fs project for information about putting this kind
of functionality into Linux.
--
Martin Pool, Linuxcare, Inc.
+61 2 6262 8990
[EMAIL PROTECTED], http://www.linuxcare.com/
Linuxcare. Support for the revolution.
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