Hierarchical Storage

Hierarchical Storage

When looking at the data flow when doing a backup, the data passes through a collection of components, each with its own speed. Looking at Figure 9, we see that the data moves from the hard disk to the tape over many different types of components. The slowest one is the tape drive, at 400 KBps. This means that a bottleneck often occurs here.

Figure 9. Component Speed

Using newer tape technologies would improve transfer rates (up to 5 MBps for DLT 7000 technology), but unfortunately, this is not the only factor that makes the tape device slow. Tape mounts, for example, can take up to several minutes. Even more when human intervention is required.

To solve this bottleneck, we could include a buffer mechanism just before the tape drive. This can be done by inserting a fast storage mechanism, for example a hard disk. This hard disk will then serve as an intermediate storage place, before transferring the data to the tape drive. This is what we call a hierarchical storage mechanism.

What will happen now when we take a backup? The backup data is backed up to the hard disk. Since its transfer rate is much higher, and no mounts are necessary, the bottleneck effect disappears. At one point, though, data must be moved (or migrated ) to the next storage device, in our case the tape. This data movement can occur in two ways:

The most user friendly way is the system-triggered way. A possible way to do this would be to define a high and a low threshold. When the amount of data on the hard disk to which we are backing up reaches the high threshold, a process is started that migrates data to the tape. This process stops when the low threshold is reached.

Hierarchical storage can also be used to minimize tape mounts during backup when tape collocation (see Incremental Pattern) is used.

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