In the previous section, we looked at the deficiencies of tape. In this section, we will explore the pros and cons of disk staging as an interim step to fully implementing disk-based backup.
Since disk has historically been too expensive compared to tape, most IT organisations found a middle ground by using a small amount of disk between the backup/media server and the tape library to hold the most recent one to two weeks of backup data.
This allowed for faster and more reliable backups, as well as faster and more reliable restores. However, keeping more than two weeks of onsite backups on disk would be cost-prohibitive.
Example: Assume a Monday through Thursday rotation with:
- Full backups of email and databases each night
- Incremental backups on files, where only the files that have changed since the last backup are backed up each night
If the nightly backup is 5TB (about 25 per cent of the full backup) and the weekend full backup (equal to the primary storage) is 20TB, then keeping one week of backups would require:
5TB x four nightlies (20TB) + the weekend full (20TB) = 40TB
To keep two weeks of backups for the same 20TB of primary storage, you would need 80TB of disk. For each additional week, you would need to add another 40TB. This is the reason why disk staging only augments tape but does not replace it, and why most organisations keep just one or two weeks of backups using disk staging and then put all the rest of the onsite retention and offsite disaster recovery on tape.
Disk staging is a pragmatic interim step and has eliminated some of the challenges of tape, but it is nonetheless a band-aid and has not eliminated all of the problems. Past the second week of retention, tapes are still used onsite and transported offsite for disaster recovery. Since tape is still used both onsite and offsite, the IT organisation has to deal with the same failure rate issues, security challenges, IT management burden and downtime concerns .
The ideal goal is to use disk exclusively, but to do so at the cost of tape.
This guide explains the various backup complexities, enabling you to ask the right questions and make the right decision for your specific environment and requirements. Stay tuned for the next part of this guide, which will be live on ITProPortal shortly.
Bill Andrews is the president and CEO of ExaGrid Systems.