The disk-to-tape shuffle
Whether they're too fast or too slow, tapes present a challenge for users migrating their data onto high-speed SANs. Over the past year, however, they've gotten a new option with the rise of storage arrays designed specifically for online backup, which feature scads of slow IDE disks and often support tape control languages so they can appear to the network as nothing but a fast tape drive. Some market leaders are Network Appliance's NearStore R100 (from 12TB to 96TB) and R150 (offering 12TB or 24TB each), Quantum's DX30 (3TB of RAID5 storage) and StorageTek's BladeStore (from 4TB to 160TB).
Deciding how much disk to add to existing tape drives requires intuition and experimentation (see "ATA disk outperforms tape"). It's important to consider, for example, how much data needs to be online and available all the time, or how much simply needs to be put onto tape and archived for legal reasons. Trying to keep everything on disk will soon prove frustratingly difficult, but putting it there with the intention to eventually move it to tape can provide much-needed flexibility.
In most cases, such disk arrays are being positioned not as replacements for tape but as a sort of staging area to be interjected between the production environment and tape. Using conventional snapshot techniques, data can be quickly duplicated and then backed up at the user's convenience: "We could make a copy to disk," says DDPSC's Belfield, "then back up that copy [to tape] and, if it took a week, it wouldn't be an issue because we'd still have a copy of it."
It's also possible to replicate the slow-fast duality of a tape-disk solution using tape exclusively. StorageTek, for one, balances speed and capacity between its 9840, which stores 20GB per cartridge and offers four-second load times and 12-second average seeks; and the 9940, which offers 60GB or 200GB uncompressed per cartridge, but has an 18-second load time and 51-second seeks.
If you need to support hierarchical storage management (HSM), you could easily install both to provide a tape-only infrastructure in which data is moved to fast tape first, then transferred onto the slower cartridges for archiving. Disparities between the performance of installed tape drives become irrelevant, however, when a near-line disk is added to the mix.
The approach you choose depends on your performance needs and backup windows. "[With tape] you still have the benefits of a much lower-cost storage medium than a disk system," says Pam Baker, advisory systems integration engineer with StorageTek, in Louisville, CO. "Yet near-line storage has added another option for customers to choose from; it can provide a fast-access buffer that provides enhanced functionality in the overall backup architecture."
