Disk versus tape: performance considerations
Disk is now a common backup medium. Backup data on disk generally provides faster restores.
Tuning disk-based storage for performance is similar to tuning tape-based storage. The optimal buffer settings for a site can vary according to its configuration. It takes thorough testing to determine these settings.
Disk-based storage can be useful if you have a lot of incremental backups and the percentage of data change is small. If the volume of data in incremental copies is insufficient to ensure efficient writing to tape, consider disk storage. After writing the data to disk, you can use staging or storage lifecycle policies to copy batches of the images to tape. This arrangement can produce faster backups and prevent wear and tear on your tape drives.
Consider the following factors when backing up a data set to disk or tape:
Short or long retention period
Disk is well suited for short retention periods; tape is better suited for longer retention periods.
Intermediate (staging) or long-term storage
Disk is suited for staging; tape for long-term storage.
Disk is better suited to low volume incremental backups.
Synthetic backups
Synthetic full backups are faster when incremental backups are stored on disk.
Restore from disk is usually faster than from tape.
Multistreamed restore
Must a restore of the data be multistreamed from tape? If so, do not stage the multistreamed backup to disk before writing it to tape.
Speed of the backups
If client backups are too slow to keep the tape in motion, send the backups to disk. Later, staging or lifecycle policies can move the backup images to tape.
Size of the backups
If the backups are too small to keep the tape in motion, send the backups to disk. Small backups may include incrementals and frequent backups of small database log files. Staging or lifecycle policies can later move the backup images to tape.
The following are some benefits of backing up to disk rather than tape:
Backups to disk do not need to be multiplexed. Multiplexing is important with tape because it creates a steady flow of data which keeps the tape in motion efficiently (tape streaming). However, multiplexing to tape slows down a subsequent restore.
More information is available on tape streaming.
See NetBackup storage device performance in the data transfer path.
Faster access to data
Most tape drives have a "time to data" of close to two minutes. Time is required to move the tape from its slot, load it into the drive, and seek to an appropriate place on the tape. Disk has an effective time to data of 0 seconds. Restoring a large file system from 30 different tapes can add almost two hours to the restore: a two-minute delay per tape for load and seek, and a possible two-minute delay per tape for rewind and unload.
With tape-based systems, full backups must be done regularly because of the "time to data" issue. If full backups are not done regularly, a restore may require too many tapes from incremental backups. As a result, the time to restore increases, as does the chance that a single tape may cause the restore to fail.