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Overwrite versus append of media
Overwriting and appending to cartridges is also a concept where virtual tape has a benefit. With physical media
it is often sensible to append multiple backup jobs to a single cartridge in order to reduce media costs; the
downside of this is that cartridges cannot be overwritten until the retention policy for the last backup on that
cartridge has expired. The diagram below shows cartridge containing multiple appended backup sessions some
of which are expired and other that are valid. Space will be used by the D2D to store the expired sessions as
well as the valid sessions. Moving to an overwrite strategy will avoid this.
With virtual tape a large number of cartridges can be configured for “free” and their sizes can be configured so
that they are appropriate to the amount of data stored in a specific backup. Appended backups are of no benefit
because media costs are not relevant. In addition, there will be a penalty when performing any tape offload
because the whole cartridge is offloaded with all backup sessions.
Our recommendations are:
Target full backup jobs to specific cartridges, sized appropriately
Reduce the number of appends by specifying separate cartridges for each incremental backup
Taking the above factors into consideration, an example of a good rotation scheme where the customer requires
weekly full backups sent offsite and a recovery point objective of every day in the last week, every week in the
last month, every month in the last year and every year in the last 5 years might be as follows:
– 4 daily backup cartridges, Monday to Thursday, incremental backup, overwritten every week.
– 4 weekly backup cartridges, Fridays, full backup, overwritten every fifth week
– 12 monthly backup cartridges, last Friday of month, overwritten every 13th month.
– 5 yearly backup cartridges, last day of year, overwritten every 5 years.
This means that in the steady state, daily backups will be small, and whilst they will always overwrite the last
week, the amount of data overwritten will be small. Weekly full backups will always overwrite, but housekeeping
has plenty of time to run over the following day or weekend or whenever scheduled to run, the same is true for
monthly and yearly backups.
Total virtual tapes required in above rotation = 25
Each backup job effectively has its own virtual tape.
The customer is also able to offload a full backup every week, month and year after the full backup runs to
physical tape for offsite storage.