On 10/16/00 01:12:02 PM +1100 Martin Pool <[EMAIL PROTECTED]> wrote:
+------
| 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.
NetApp's take a snapshot when their nvram is full or IIRC 10 seconds have
passed.
| 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.
|
+-----X8
Legato Networker checks the file metadata after taking a backup and then
takes another backup if the file has changed, it takes a makimum of 3
copies. This seems like a reasonable approach for rsync to take.
/Michael
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