HI Tab, I think "it depends".
When I was using it, the answer was yes. What we used it for was squishing a lot of .pst files. They tended to be fairly large, up to 500m, and were excellent candidates for subfile, because only a small % of the .pst file actually did change each day. (Besides benefits to occupancy, with a lot of clients in the config it reduces the requirements for your disk pool, the load on your network, etc.) With subfile on, and 3 versions, the first backup (of a file) after you turn on subfile will still be the entire file, because it has to create the base file in the cache directory. So don't be surprised that you get no benefit on the first run (and don't forget to exclude the subfile cache from backups!) The next 2 versions sent from the client will just include the BLOCKS that change. And the server just records the number of BYTES that the client sends to it, so the recorded occupancy will drop, and the actual occupancy will drop. However, with only 3 versions, you many not get that large a % of benefit, because remember that even when you get to the 7th version of a file, the server is still holding on to the base, plus 6 deltas - there always has to be a base file in server storage, which is full size. So in your case, you are getting rid of less than 2 versions, in terms of bytes saved. The benefit would be higher, if you were starting out with more versions in server storage. And remember, - very small files don't qualify for subfile - very LARGE files don't qualify for subfile - if a large enough percentage of the file changes, it does a full (of that file) anyway. So I have no doubt that you can construct cases, where the answer is YES or the answer is NO, depending on the data. On the other hand, it's a harmless thing to test out. Turn it on for just one of your servers, and monitor your occupancy. You should have a good idea of the impact in a week or so! Caveat: DON'T EVEN THINK ABOUT IT, unless you are using collocation! A restore requires base plus deltas - you don't want to do multiple tape mounts to get 1 FILE back! Wanda Prather "I/O, I/O, It's all about I/O" -(me) -----Original Message----- From: ADSM: Dist Stor Manager [mailto:[EMAIL PROTECTED] On Behalf Of Tab Trepagnier Sent: Friday, November 04, 2005 9:55 AM To: [email protected] Subject: Does subfile backup reduce tape occupancy? Like lots of users, our tape libraries are overloaded and we're looking for ways to reduce what we're keeping before spending money on new equipment. Server: TSM 5.1.10 on AIX 5.2 Clients: mostly TSM 5.1.7 on Windows XP/200x. Storage: mix of disk, 3570, and LTO Most filesystems are configured to maintain three versions. If I have a server whose current occupancy is 1000 GB, and I turn on subfile backups with the same number of versions, will the actual occupancy go down? Or will it remain the same? Thanks. Tab Trepagnier TSM Administrator Laitram, L.L.C.
