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IBM-AUSTRIA - PC-HW-Support 30 Aug 1999 |
Single Tape Devices and Libraries
Single Tape Devices and Libraries
You will also need to make a choice between single
tape devices (or manual) or tape libraries. A manual device requires the
operator to insert and remove tapes manually. A tape library can hold a
certain number of tapes, and load them automatically. These libraries
can also make use of more than one tape device, increasing throughput
and functionality.
It is quite simple to determine your needs in this
case. When automating the backups to run at night, you should be able
to do this without manual intervention (unless you have operators who
can mount tapes). This means that the backup should hold one tape. This
can easily be determined if you are using a full backup pattern (see
Full/Incremental Pattern
and
Full/Differential Pattern),
but could get difficult if
you're using the incremental pattern, because you do not have a clear
vision of tape usage/reusage.
Besides the enlarged capacity, libraries
can also be used to install a low maintenance backup solution. This
could mean that you could hold a series of backups in your libraries,
lowering the need for operator intervention.
Another point
to consider is the total number of backup copies which should be stored
(in other words, the expiration time of a backup). For several reasons
it is a wise decision to keep more than one backup generations:
- Any
backup may fail. Even so-called verify options (if provided by the
backup software) cannot detect all types of failures and
inconsistencies. For example, for the most databases is no way to
compare byte for byte the tape contents with the database files because
the database is permanently modified in the background, and the special
database backup agents can ensure a 'frozen picture' only during a
small time window.
- The reason for a restore may be a software or human
error which was detected some days after the last full backup. If this
is the only complete backup copy available then the situation becomes a
nightmare.
- A larger disaster in the datacenter may damage some or all
cartridges there, or they are not accessible for disaster recovery at a
remote site. Thus there are rules established for moving backup copies
of business-critical data to separate and safe locations. This leads to
a dilemma: such copies are in a very safe place, but it is not
acceptable that for restoring a deleted WinWord document you need to
visit a bank safe. Working with multiple backup generations gives a way
to balance the requirements.
Using single tape drives, the consequence
is simple: Buy enough cartridges. Do not try to save money here - even
multiplying the cartridge stock is much cheaper than the financial loss
in critical situations.
For tape libraries, you have to decide how many
backup generations will be kept in the library itself. This may change
the choice of the library model dramatically, since you will need to add
these copies to the storage capacity needed.
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