Making it work: Tip #2

One of my pet hates of Backup Exec is it's rather poor disk space management for Backup To Disk Folders. There are 2 approaches to Disk Space Management, the Sane Way, and the Stupid Way.

Guess which Backup Exec chooses...

As you can't set a limit on the amount of space a Backup To Disk volume can occupy (almost like a LTO-1 tape has a 100Gb storage capacity, and that's that, if you run out, you run out), you'd expect to be able to do similar on Backup Exec. But while you can tell it how big an individual Backup To Disk FILE is, you can't set a limit on the total space a "Disk" can occupy. The limit of course is the physical space on the drive itself.

The "workaround" is that if you had a drive, let's say it's 500Gb, and you have 6 Backup to Disk "Folders" on that drive. Set each one to have a Disk "Reserve" of something (the default is "nothing", naturally), let's say 10Gb. When you first use Backup Exec, it'll just keep making "B2Dnnnnn" files, but once you get to <10Gb left on the PHYSICAL drive, the Backup Exec Folders will then theoretically start to be overwritten, preventing the problem of running out of space completely.

Follow that? No. OK, try this:

PHYSICAL DRIVE (we'll call it Drive E:) has 500Gb Space, and it's only being used by Backup Exec.

You have 6 folders, each set with 10Gb "Disk Reserve":

Folder A, Folder B etc

Roll forward 2 weeks...

Folder A has 40Gb
Folder B has 2Gb
Folder C has 19Gb
Folder D has 125Gb
Folder E has 1Gb
Folder F has 28Gb
Folder G has 6Gb
...and the physical drive has 279Gb Free.

Roll forward 4 weeks....

Folder A has 100Gb
Folder B has 10Gb
Folder C has 90Gb
Folder D has 160Gb
Folder E has 2Gb
Folder F has 118Gb
Folder G has 10Gb
...and the physical drive has 10Gb Free.

At some point between weeks 3 and 4, the physical space hit 10Gb, so old "media" was overwritten. That's the theory of how this works.

Here's the reality:

* Make sure the PHYSICAL storage is MORE than the expected DATA need - that's full backups, storage for daily changes, incrementals, synthetics and so on, plus any other data you have on the same volume (perhaps Catalogs). It pays to have perhaps 50% more storage than you truely need, and more if you can.

* Make sure your storage calculations are enough for the retention. So if you've got a 4 week retention, do a weekly full, you need x 4 plus incremental storage. Realistically, maybe 6 times the storage.

* If your overall data storage decreases, don't expect more space to appear on the phyiscal drive. Because it treats "media" like a tape, it handles it the same way. If you had 30 tapes, and you only needed 20 of them now, the other 10 don't get erased from the earth, they just stay kicking about in your cupboard. Backup exec keeps your virtual "tapes" in it's cupboard. (we'll explain how to manage this better some other day).

* Regularly monitor your Backup to Disk Folders, particularly once you're getting to the space limits, because if your reserve is too big, and the retention period longer than you can cope with in physical space terms, you'll start getting failed jobs if there is simply no overwritable media left to be used.

* Having "overwriteable" media available is handy in some ways, as it means data beyond the retention period can still be available if the media hasn't yet been overwritten, almost building in a "last chance saloon" retention period, but it is also likely to consume all available space.

* If not all data is equally important, consider having different backup to disk folders, so that more critical data can be given longer actual retention times, but don't forget overwriteable media in one folder can't be claimed by another to add space, so good planning of physical space, folder space and necessary space is required.

If you follow this guide you can reduce the misery of the lacklustre Backup Volume Management. It won't allow you to have absolute limits for a media set (which you'd no doubt want), but it does stop you just running out of space.