To improve backup performance to tape, backup software will gather data from multiple job streams, typically 15 or more, and then interleave the data into a super block, which is then sequentially written to tape.
To recover a single application, you must read all the data from the tape, stripping away 14 out of 15 records to get the one you need; this is multiplied by the need to read through numerous super blocks associated with sequential files that often are spread across 30 or more tapes. This has obvious performance issues, and is prone to failure from media and transport reliability issues, which have no level of redundancy whatsoever. If you have an uncorrectable read error, you can lose the entire backup.
Protection objectives are measured as Recovery Point Objectives (the amount of data at risk) and Recovery Time Objectives (the amount of downtime you can tolerate); they’re a concern with tape. Tape can easily take nearly four days to recover 10 TBs for a Linear Tape-Open 3 (LTO-3) as compared to 2.5 hours for a disk used as a protection library. What’s your cost of downtime?
When using disk as a protection library, the problem is solved. Backup software can index all the data as it stores it directly to disk. Whether you have to recover a single sub-object to VMware, an email message to Exchange, or a SharePoint Document, you can individually recall them. Because it’s a random access recovery, it’s fast. The Network Data Management Protocol (NDMP) lets you write directly to disk for easy configuration and management of network-based backups. With NDMP, network congestion is minimized because the data path and control path are separated.
With disk used as a protection library, backup can occur locally—from file servers direct to disk, while management can occur from a central location. Because it’s indexed by the backup application directly to disk, it’s simple. Because of the decreased infrastructure complexity, it’s easy and more efficient.
Client encryption can protect the data all the way through the network into the backup server and onto disk. It’s a faster restore that’s easier to manage and occurs at or below the cost of tape.
There are many compelling reasons to use disk over tape as a protection library—chief among them the ease of managing and using disk.
Managing tape cartridges is complex. Many users complain they can’t recycle backup tapes fast enough—forcing them to constantly buy more media. Backup typically uses a Grandfather-Father-Son (GFS)-managed retention plan. The backup schedule generally will include daily, incremental backups and weekly, full backups. If you look at what happens over the course of a year, every TB of primary disk causes 25 TBs to be written to tape to protect it. The cost to implement, maintain, and manage this is extreme. If you were backing up 42 TBs of disk, over the course of a year, you’d need 6,300 LTO-2 tapes. This assumes an 80 percent efficiency usage for each cartridge. At $26 per cartridge, the cost is $163,000, and you have 25 copies of the data to manage and maintain.