Storage Area Networks For Dummies®

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Part III: Using Advanced SAN Features
How many drives do you need to back up 200GB in three hours? If each
drive can back up 30GB per hour at 2:1 compression, divide 200GB by 30GB
to figure out how many drives will be needed to fit within the three-hour
backup window.
200GB / 30GB per hour = 6.6 hours
One drive can back up 200GB in 6.6 hours. To fit within the three-hour
window, you need three drives (6.6 hours / 3 hours = 2.2 hours).
You can use this formula to figure out how many drives you need for any
speed drive that you want to purchase. Just change the GB-per-hour number
for the speed of the drives you’re using. If you buy drives that go twice as fast
as DLT 7000s, you need only two (1.5 rounded up to 2) to back up the same
amount of data in the preceding example.
If you’re using tape libraries that can replace tapes automatically when they
become full, factor in about a 7-percent reduction in backup speed: the time it
takes to replace the tapes in the drives in the library when the tapes are full.
Also, each drive can back up only a single server at a time. The more drives
you have, the faster you can back up all your servers.
Restore times will be about twice as long as backup time. During restore, the
data is read sequentially from the tapes, which is slower than reading the data
from disks during backup.
By the way, many tape drives are rated not in gigabytes per hour (GB/hr)
but rather in megabytes per second (MB/s). Some are listed as megabytes
per minute (MB/min). To determine different data rates, you can use these
formulas:
Conversion
Formula
Convert MB/s to GB/hr Multiply by 3.6
Convert GB/hr to MB/min Multiply by 16.66
Convert MB/min to MB/s Divide by 60
You can use these formulas to calculate the differences between how the
tape drives are rated by the manufacturer to the throughput you will get in
GB per hour to determine how many drives you need to get back up all your
data in the required time.
For more information on using snapshots or other Point-in-Time solutions
for backup, see Chapter 10. For more information on newer backup methods
such as CDP, see Chapter 14.