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10.5: One way to use Time Machine and a backup server
Authored by: ikioi on Dec 24, '07 12:42:11PM

There's only three possible issues that I see.

The Finder in 10.5 provides an extended interface for permissions on files. The old 10.4 "Get Info" window just let users set regular unix owner/group/world read and write permissions. These days (10.5), you can list more than just owner/group/world and what you set in the "Get Info" window is stored as an ACL on the file or folder. In 10.4 ACLs were off by default and almost never got created on a normal machine. in 10.5 they are on by default and likely to end up at least occasionally created by users even if those users have no idea what ACLs even are.

Time Machine (TM) is extremely clever at handling ACLs. When a file or folder with ACLs is backed up in TM, all of the ACLs are shifted by one spot (ACL #0 becomes ACL #1, and ACL #1 becomes ACL #2, etc) and a new ACL is created in slot #0. The new ACL denies permission to delete or modify the file in any way. That's why you can't delete files from your TM backup using Finder.

So all of this is great in TM, but the only question is whether rsync properly copies ACLs. There are a couple of way rsync might not (I'm not sure). It might fail to copy them at all (which means that some permissions set by users in Finder would disappear, and the backups on the server could be unwittingly modifed) or if rsync does copy Apple ACLs, it might do a breadth first copy instead of depth first meaning that a folder might get the "deny all changes" ACL set before the child files or folders are copied.

I don't know if any of those are going to be problems, but there's a chance. The mitigating factors are that TM may use rsync as it's backend anyway, which would mean you're set and even if it doesn't, of the above two problems you might not be too worried about the first one, and you'd probably notice the second one right away because almost nothing would back up to the server.

This is a more serious issue and it simply depends on whether some bugs in 10.4's rsync have been fixed in 10.5. In 10.4, rsync would copy extended attributes and resource forks and such, but if you did rsync -aHE, then any file with a resource fork and more than one hard link would silently be skipped. No error, no notice at all, the file just wouldn't be created on the target. I recall the behavior was more subtle if the file already existed on the target. If you haven't already checked it, it's worth making a file with a resource fork, backing it up with TM, modifying its parent folder, and then backing up the file again with TM (to make sure the file has more than hard link), and the finally, trying to rsync the TM backup to an empty target location and seeing if that file is skipped by rsync.

This one won't kill you but might make the server copy take more space than TM copy. TM uses hard links to folders, but I don't know if the API to do that is publicaly available because hard links to folders are dangerous if used improperly. I know the ln command _cannot_ create hard links to folders in 10.5 . I don't know if rsync can or not. If it can't, then it wouldn't even know to check for hard links, so it would still behave gracefully, just creating lots of duplicate entries for folders and files on the server that TM didn't have to on it's local partition. It means the server drive will be slower on disk lookups, but it shouldn't break anything.

Anyway, that's my 2 cents. I can't think of any other problems with it. Nice idea!

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10.5: One way to use Time Machine and a backup server
Authored by: ikioi on Dec 24, '07 12:43:45PM

Woops . Looks like matsw already mentioned my ISSUE 3. Sorry for the duplication. :-)

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