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How many tapes of which sizes should I use?

Posted: 10 Nov 2010, 08:48
by jsf
With Firestreamer file media, you can easily choose the number and size of virtual tapes to suit your needs. When doing so, consider the following:
  • Firestreamer is unaware of the nature of data written to virtual tapes. The optimal number and maximum size of tapes ultimately depend on the logic built into your backup software, the actual capabilities and performance of your storage and network, and your backup policy requirements.
  • It’s entirely up to the backup software to write particular data to a specific tape. Firestreamer has no control over it whatsoever. Moreover, the backup software may write only a limited amount of data to a tape then switch to the next one, instead of filling the tape until it's full.
  • Microsoft DPM may not work well with a small number of tapes. It may quickly consume all the tapes, not necessarily up to their full capacity, and then ask for a free tape.
  • Huge tapes (hundreds of gigabytes in size) are inconvenient to maintain. Imagine that you need to copy such tapes across a network, or to a smaller drive.
  • The smaller the tape, the lesser the amount of data that can potentially be lost if the tape file is corrupted or accidentally deleted.
  • If the tapes are too small comparing with your typical backup, the bigger number of tapes per backup may affect the overall performance because of the overhead associated with the backup software switching the tapes.
  • Firestreamer has the hard limit of 8,388,607 MB (7.999 TB) per tape for both file media and drive media. It does not however limit the size of your backups as backup software is usually able to span tapes.
  • We found that some storage devices are unable to properly handle big files. If you are experiencing problems with the files more than several gigabytes in size, such as reduced performance or high CPU usage, try updating the drivers and/or firmware of the storage device, or try a different model or brand.
For example, if you have a 2TB external disk drive, you may want to follow the below procedure.
  1. Calculate the number of tapes to create on the drive as TN=(ST-SR), where ST is the total number of storage slots in the library, and SR is the number of reserved storage slots. In the default configuration of Firestreamer, ST=200. The reserved storage slots normally remain free, in case if you need to load more tapes, for example, to perform a restore. Let's choose SR=50. Then TN=(200-50)=150.
  2. Calculate the maximum size of each tape as TS=(DT-DR)/TN, where DT is the amount of available disk space, DR is the amount of reserved disk space (usually, 5-10% of DT), and TN is the number of tapes calculated at step 1. In our case, TS=(2TB - 10%)/150=12GB.
Note that the space actually available on a file system (DT) may be less than the amount displayed by Windows Explorer because of the space reserved for Recycle Bin, quotas, shadow copies and other system information. You may want to fill up the destination storage to verify the actual value of DT.

If you are unsure of how many tapes of which sizes to create and load into Firestreamer for your first backup, try the following. Use Firestreamer in its default configuration (one library, five tape drives, 200 storage slots). Create and load 150 tapes, but don't limit their sizes. Run backups for some time, and see how they go. In particular, see how many tapes are actually used by your backup software, and what actual size they reach. Based on that information, you will be able to fine-tune the settings as needed.

With Firestreamer drive media, the size of a tape is equal to the size of the physical medium. Refer to the above to choose the number of tapes.