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:
- System triggered
- User initiated
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
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