Capacity

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Drives

Now that we have the amount of data and the number of hours needed to store that data, all we have left to do is some basic math. Just take the total amount of data that has to be backed up daily and divide by the duration of the daily backup window:

Ideal data transfer rate = Amount of data to back up ÷ Backup window

If you have 100 GB of data and an 8-hour window, your ideal data transfer rate would be 12.5 GB/hr.

After you have an idea of the ideal data transfer rate, you can then look at the different drive types to see which might offer the best fit for your needs. Not surprisingly, this is a little more complicated than just looking at the base numbers, though. With potential drive technology, you must consider both performance and capacity. In larger enterprise environments, one size usually does not fit all. As mentioned several times, you need to look at the recovery requirements first and work back. This might mean you will need two different types of drives, some that are very high performance but with less capacity and some that offer higher capacity with lower performance. Data that is being kept for long retention periods, especially to fulfill legal requirements, might be better suited for the lower-performance but higher-capacity media. Data that might be required for immediate restores where time is money might be better suited for the high-performance media. It is not uncommon to have backups done to high-performance drives and media and then the images vaulted to high-capacity drives and media for off-site storage.

A sample of tape drive transfer rates, capacities, and access times is given in Table 4.1. This information can be very helpful in determining which drive technology you need, but never forget these are all theoretical numbers and are given without taking into account the internal drive compression. Drive manufacturers advertise compression rates for the different drive technologies. These vary depending on the drive but are also theoretical numbers. These specifications can change with new firmware levels or versions of the drives. To get the most accurate numbers, contact the drive vendor or go to their Web site, where you'll find up-to-date specification sheets.

Table 4.1: Tape Drive Data Transfer Rates and Capacities

DRIVE	THEORETICAL TRANSFER RATE GB/HR (NO COMPRESSION)	THEORETICAL CAPACITIES GB (NO COMPRESSION)	ACCESS TIME EXCLUDING LOAD TIME	COMPRESSION
4mm (HP DDS-2)	1.8	4
4mm (HP DDS-3)	3.6	12
Mammoth	11	20	60 sec	2:1
Mammoth-2	42.4	60	60 sec	2:1
DLT 4000	5.4	20	68 sec	2:1
DLT 7000	18	35	60 sec	2:1
DLT 8000	21.5	40	60 sec	2:1
SDLT	39.6	110	70 sec	2:1
9840	36	20	11 sec	2.5:1
9940	36	60	41 sec	3.5:1
LTO	52.7	100	25 sec	2:1
AIT-2	21.1	50	27 sec	2.6:1
AIT-3	42	100	27 sec	2.6:1

When you start actually figuring how many of which kind of drive you will need, we recommend using the native transfer rates and capacities without compression. It is very difficult to estimate what kind of compression rate you will experience, as it is totally dependent on the makeup of your data. Some data is very compressible, while other data will yield very little compression. If you do your architecture based on no compression, the only surprises you should experience should be good ones; you will have plenty of capacity with room for growth.

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