6.5 SAN Backup Devices

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6.5.1 Tape Libraries

The device of choice for SAN backup and restore is the tape library. A tape library is a tape hotel that holds multiple tape cartridges and one or more drive mechanisms. Under robotic control, tapes are moved from storage slots to drives for reading and writing.

The drives in the library are typically DLT4000s, DLT7000s, DLT8000s, or HP9840s. This variety in drive types allows for tradeoffs between capacity and speed. You can have one or the other, but not both, in a drive. However, you can mix drive types in some tape libraries.

When you see tape libraries with paired numbers (such as HP s SureStore E 2/20 or 10/180 models), the first number refers to the maximum number of drives in the unit. The second number refers to the maximum number of cartridges the unit can hold.

At the low end, there are libraries that rackmount in only 5 rack units (U) of space or fit on a desktop. One such model (not made by HP) has one mechanism and 22 DLT cartridges, producing a capacity of 1.76 TB. The manufacturer says the library gives you the capacity you need for a full month of hands-off backup, which is odd advertising, since there s no way the manufacturer could guess at your monthly backup requirements.

Hewlett-Packard makes tape libraries for midrange and high-end environments. HP midrange tape libraries include:

HP SureStore E Tape Library 2/20
HP SureStore E Tape Library 3/30
HP SureStore E Tape Library 4/40
HP SureStore E Tape Library 6/60
HP SureStore E Tape Library 6/100
HP SureStore E Tape Library 10/180
HP SureStore E Tape Library 10/588
HP SureStore E Tape Library 20/700

Models 2/20, 4/40, and 6/60 are newer , based on the same architecture, and can scale from one drive (in the 2/20) to six drives with 20 to 60 cartridge slots. That produces a range of 1.6 TB to 4.8 TB, assuming 2:1 data compression.

Approaching the high end of the midrange is the SureStore E Tape Library 10/180, a newer HP offering that could work well on a SAN. It comes with up to ten DLT8000 drives or six HP9840 drives, and has a maximum capacity of 14.4 TB. It will probably be offered with LTO in the future.

For those who believe there is no substitute for immense capacity, the high-end HP tape library is the 20/700. It has a capacity of about 56 TB, assuming 2:1 compression.

This tape library has some of the high-availability features previously discussed for disk arrays: dual fans, hot-swappable tape drives, hot-swappable power supplies , and dual pathing. That makes it a very suitable candidate for a SAN.

What s the difference between a 20/700 with DLT8000 drives and HP9840 drives? Capacity versus speed.

Table 6-3. DLT8000 vs. HP9840 Drives

	DLT8000	HP9840
Number of drives	20	12
Cartridge capacity	40 GB	20 GB
Library capacity	28 TB (native 56 TB (compressed)	14 TB (native) 28 TB (compressed)
Throughput	216 GB/hr (native 432 GB/hr (compressed)	360 GB/hr (native) 720 GB/hr (compressed)

This scale of tape library would be a good match for backing up data from a high-end disk array, such as HP s XP256 (Figure 6-6).

Figure 6-6. 20/700 and XP256 in a SAN

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