Q

Does the backup window still exist?

Traditional direct server attach of tape creates the concept of a "backup window" and so transfer rate becomes a critical factor when choosing a tape drive. What I am unclear on is whether the backup window still exists in a SAN environment? Should I look just at capacity?

A backup window will still apply in a SAN environment if you still use the network for backup. SAN based backup has come a long way in the last few months. The whole idea here is to never use the public LAN for backup traffic, and run at SAN speed as opposed to LAN speed to do backup. The wonderful thing about SAN backup is its block based, there is no IP overhead, and it allows you to actually "stream" the tape drives. This means the tapes do not go into the "shoe-shine" effect, where the tape moves back and forth across the tape head to reposition itself for the next data segment. If you meet the "feed-speed" of the drive, you can get great throughput. If using libraries, with multiple drives, you can achieve the maximum performance spec for the devices. I have found DLT devices to get about 20GB per hour per head. LTO gets much faster throughput, but SuperDLT is catching up, and is backward compatible to older tapes.

Traditional SAN backup techniques required the backup engine to reside on every server in the SAN, so it could back-up it's own LUNs. Newer versions allow a dedicated backup server to mount BCVs of production volumes, and back up the BCVs to tape. This allows "NO-IMPACT" backup to production apps, and removes the backup window, since backup can be done anytime after the BCV (Business Continuance Volume) is created.

The latest SAN backup method allows for "serverless" backup, using the extended copy command in the fabric. This can be done through an intelligent library, or through a "data-router" in the SAN path. The data-router effectively asks the production server whether it can mount a volume, and then "copies" the data directly from disk to tape. A backup server is usually still needed though to store the index files. All the major backup software vendors currently support Xcopy (or Ecopy) with their solutions (CA, Legato, Veritas). Call your vendor for specific support for OS versions (NT AND Unix).

Chris

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This was first published in November 2001

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