Virtual Media—How Many and What Size?

There is no correct answer to this question as it really depends on your backup regime and the backup application in use. A DXi system does not preallocate disk space when a virtual tape cartridge is created, there can be an inclination to create a lot of large virtual tape cartridges, greater than the amount that is actually required. While this has no initial bearing on the DXi’s capacity utilization, it can cause issues down the track.

Regarding the quantity of virtual media you need to create, as long as there is sufficient media to hold the amount of backup data for the required retention period, the quantity does not really matter. However, it is extremely important, as mentioned earlier, that the aging and expiry rules of the media pool are defined early. If these rules are not defined, the virtual tape cartridges will never enter the scratch pool resulting in you running out of virtual media, and in turn space on the DXi system. As a rule of thumb, more is better than less; as long as expiry rules are defined.

Although there is no perfect size when it comes to creating virtual tape cartridges, there are a few factors to consider. If a virtual tape cartridge is created at 200 GB, then it will be presented as storing 200 GB of native data.

Creating smaller sized media is preferred, for example 50 GB or 100 GB, as it allows a virtual tape cartridge to be completely filled even when backing up smaller data sets. Smaller sized media also aids the transfer of data from virtual to physical tape platforms. Once again, virtual tape cartridge size will depend on the backup application in use.

It is important to note that the design of the DXi will not allow a virtual tape cartridge to be written to it if its size is larger than the amount of free space available on the DXi. For example, if you have a virtual tape cartridge size of 400 GB but only 300 GB of free space is available on the DXi, the backup will fail when attempting to write data to the virtual tape cartridge. As a result, smaller virtual tape cartridge sizes are better.

Remember, the DXi system is always cautious of being filled up so that the integrity of stored data is not compromised.