Backup systems, actual hosts, and logical hosts

April 30, 2014

One of the little but potentially important differences between backup systems is whether they can back up logical hosts or if, for one reason or another, they can only back up actual hosts. Since this sounds like a completely abstract situation, let's set up a concrete one.

Let's suppose that you have three fileserver hosts, call them A, B, and C, and two logical fileservers, fs1 and fs2 (and some sort of movable or shared storage system behind A, B, and B). Actual filesystems are associated with a logical fileserver while each logical fileserver is hosted on a particular machine (with one left over for a spare).

If your backup system will back up logical hosts, you can tell it 'back up fs1:/a/fred and fs2:/b/barney', have this work, and have the backup system associate things like index metadata about what file is in what backup run with these logical names. This is what you want because it means your backup system doesn't care which physical host fs1 and fs2 are on, which in turn makes it much easier to move fs1 from A to C in an emergency. However if your backup system insists on dealing with real hosts then you must tell it 'back up A:/a/fred and B:/b/barney', all of the index metadata and so on is associated with A and B, and the backup system will either explode or require manual attention if /a/fred ever winds up on C. This is obviously not really very desirable.

You might think that of course a backup system will back up logical hosts instead of insisting on real hosts. In practice there are all sorts of ways for a backup system to quietly need real hosts. Does the client software send the local hostname to the server as part of the protocol? Does the client software make network connections to the server and the server use the IP address those connections come from to do stuff like verify access rights, connect incoming backup streams to requested backups, or the like? Then your backup system might be implicitly requiring you to use real hosts.

(Even if the backup system theoretically copes with backing up logical hosts it may have limitations that will cause problems if two logical hosts ever wind up on the same real host or if you try to back up both the logical host and some stuff on the real host. This split between logical hosts and real hosts is a corner case and it exposes any number of potential issues.)

Written on 30 April 2014.
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Last modified: Wed Apr 30 01:19:11 2014
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