Wednesday, January 11, 2006

TSM Library Sharing

TSM is truly an Enterprise Backup tool and one of the key selling points is how well it allows you to consolidate and centralize the backup environment. One feature I have used extensively is the library sharing option. This feature allows multiple TSM servers to use the same library without conflict. It can be easier than partitioning the library, allows for dynamic allocation of tape drives, and allows for a single shared scratch pool. The key to using a shared library is creating a separate TSM instance as a dedicated library manager (whether on the same server as another TSM instance or on its own hardware). The library manager instance will not perform any backups and will not be a big instance. From my experience it will have very little impact on the physical server. When creating our library manager instance I was unsure how large to make the DB so I made it 2GB, and it currently is 4.6% utilized. The library it manages is a 9 frame 42 drive 3584. The library supports 3 TSM backup instances (2 on one server and 1 on a separate physical server) with a total of 951 clients. We put the library manager instance on the server with 2 instances, thereby creating a third TSM instance. By creating the third instance dedicated to managing the library you will have increased uptime and when you need to do maintenance or restart the instance the library manager is back up very quickly. This will not be the case if you make a production backup TSM instance the library manager due to a production TSM servers need to run expiration and usually having a larger log size making restart time take that much longer.

The other nice feature of library sharing is the ability to dynamically allocate drives to specific TSM servers. I have seen some administrators take twenty drives and path ten to one TSM instance and ten to another but this defeats the purpose. If you setup the library manager and TSM backup instances correctly then you’ll allow every instance connected to the library manager to see all the drives. This coupled with the device class mount limit setting can allow you to change drive usage on the fly. So say you have two TSM servers TSM-A and TSM-B using a twenty drive 3584 managed by a library manager instance called TSM-C. Each TSM instance sees all twenty drives but you have their device class mount limits set to ten. Now each TSM instance can only use ten drives out of the twenty. Lets say on a specific day TSM-A server is in need of more drives and TSM-B is currently only using three drives. You could update the TSM device class on each to allow TSM-A fifteen drives and TSM-B five drives.

Recently we purchased some newer hardware to replace our 9 frame LTO-1 library and are looking at using two small AIX servers using HACMP as the library controller since the new library will need to be available at all times and will be supporting at least six TSM servers. Unfortunately I have had to partition this new library due to application demands. The problem we are experiencing with this new library is that the logical portioning requires us to use the Virtual I/O option in the 3584. This option allows the library to automatically insert volumes when placed in the I/O door. This would not be a problem if the library did not then require us to assign the volumes to a specific logical library. This is done through the library and not through TSM, which adds another process to our tape management. I would have preferred to not have partitioned the library and allowed a TSM library manager instance to handle allocation of tape drives, but alas I am not able to at this time (still awaiting the HACMP instances).

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