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Disk changes everything
Incorporating disk into the backup environment seems to be every backup administrator's goal, but the best method to accomplish this is subject to debate. The options for adding disk to backup were initially limited to the rudimentary support included with backup software products. Aside from IBM Tivoli Storage Manager (TSM), which was designed around the paradigm of a disk-based storage cache, most backup products lacked optimized support for disk. This made the concept of VTLs a particularly attractive way to incorporate disk into the environment.
However, backup software vendors have scrambled to enhance the level of disk support in their products. Newer options, like EMC's NetWorker DiskBackup Option, handle disk like the random-access device it is and provide multiple, concurrent access and improved performance. In Veritas NetBackup 6.0, Symantec has added features like disk watermarks to better support the TSM-like, disk-as-cache model.
In the NAS arena, the tighter integration of Veritas NetBackup with Network Appliance has yielded more advanced disk backup capabilities like the Veritas NetBackup NAS SnapVault Option, which essentially provides long-needed integration between backup and NAS snapshot management. Not to be outdone, EMC's NetWorker offers a NAS PowerSnap Option and extends its NDMP agent to support disk as a target device. Because the use of snapshots and split mirrors has long been a popular method of
For most organizations, VTLs are the most straightforward and least disruptive option for incorporating disk into backup. And we now have enough collective experience to begin to suggest VTL best practices, including some that contradict traditional tape-only practices.
With traditional backups, it was considered extremely risky to extend the period between full backups. With disk, there's no such concern, and doing fewer full backups actually extends the capacity of the VTL, which means potentially retaining more versions of data on disk.
In the IBM TSM world, the disk storage cache was traditionally sized to accommodate at least one night of incremental backups that would later be migrated to tape. The idea was to avoid frequent disk-to-tape migrations that would slow the backup process. With VTLs, TSM still benefits from having a disk storage cache, but the sizing rule no longer applies. The cache can now be a fraction of the size and migrations to the VTL aren't a concern in most cases.
This was first published in September 2006