Disk-Assisted Backup Whitepaper

The backup window can also be reduced by using backup to disk, running many backups in parallel.
This is especially attractive for incremental backups due to the following reason:
Tape backups are fast as long as the tape drives are streaming, that is, enough data is being sent to
the drive so it does not have to stop and then re-start when it receives more data. With the ever-
increasing performance of new tape drives, the requirement on the infrastructure for backups is
increasing, too—faster networks, faster disks and faster backplanes. Especially when running
incremental backups, many parallel streams are required to feed enough data to the drives. Even
though it is copying less data than a full backup, the incremental backup can take longer to complete
due to slow disks on the client side and non-streaming drives.
To resolve that problem, backup to disk can be used as a staging area. All incremental backups are
stored on disk and can then be written to tape later in one pass, without delay, as all the data is in
one place.
Figure 1.
Increased success rate of backups completed
Using disk as a staging area can prevent backups from failing to complete due to:
• Tape drive failure
• Backups “locking” the tape drive for longer than expected and preventing other backups from
running
• Restoration jobs running in parallel
As backup to disk is not constrained to a limited set of tape drives, it is easier to accommodate
changes in the backup load and the number of parallel jobs running. This lowers the risk of backup
failure caused by tape drives not being available or drives being tied up with backup or restore jobs
that are taking longer than expected.
No or reduced need for multiplexing of backups to tape
Multiplexing is a very useful feature that allows multiple backup streams to be written simultaneously
to a tape. As mentioned previously, modern tape drives are so fast that it is almost mandatory to use
multiplexing to guarantee enough data being sent to the drive to keep it running at the highest
possible speed.
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