How to calculate the time required to back up to tape

Article:HOWTO56053  |  Created: 2011-07-25  |  Updated: 2014-06-30  |  Article URL http://www.symantec.com/docs/HOWTO56053
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How to calculate the time required to back up to tape

When you know your ideal data transfer rates for backups, you can figure out what kind of tape drive technology meets your needs. With the length of your backup windows and the amount of data to back up, you can calculate the number of required tape drives.

Table: Tape drive data transfer rates lists the transfer rates for a number of popular tape drive technologies.

Table: Tape drive data transfer rates

Drive

Megabytes per second

Gigabytes per hour, no compression

Gigabytes per hour, at 60% utilization

Gigabytes per hour, at 2:1 compression and 60% utilization

AIT-5

24

86.4

51.84

103.68

DLT-S4/S4A

60

216

129.6

259.2

DLT-V4

10

36

21.6

43.2

LTO-3

80

288

172.8

345.6

LTO-4

120

432

259.2

518.4

LTO-5

140

504

302.4

604.8

LTO-6

160

576

345.6

691.2

SAIT-2

45

162

97.2

194.4

SDLT 600/600A

36

129.6

77.76

155.52

TS1120 (3592E05)

100

360

216

432

TS1130 (3592E06)

160

576

345.6

691.2

TS1140 (3592E07)

250

900

540

1080

T10000A (STK, Sun, Oracle)

120

432

259.2

518.4

T10000B (STK, Sun, Oracle)

120

432

259.2

518.4

T10000C (Oracle StorageTek)

252

907

544.2

1088.4

T10000D (Oracle StorageTek)

252

907

544.2

1088.4

The values in the table are those published by individual manufacturers and observed in real-life situations. These values are not definitive, but should be useful for planning your requirements.

Keep in mind that device manufacturers list optimum rates for their devices. In the table, the column labeled Gigabytes per hour, no compression lists these optimum rates. In reality, it is rare to achieve those rates when a system has to deal with the following: the overhead of the operating system, CPU loads, bus architecture, data types, and other hardware and software issues. The 60% utilization columns in the table are conservative estimates, meant to approximate average, real-world performance.

When you design your backup system, consider the nature of both your data and your environment. A backup image that contains many small files may take longer to write to tape than one that contains a few large files. To be on the conservative side, use the 60% values from the table when making your estimates.

Note:

Unlike tape drives, disk devices (including VTLs) do not have a minimum streaming speed. It may therefore be a good strategy to stage slower backups to disk before duplicating them to tape: the duplication of the backup image from disk runs faster than the original slow backup.

To calculate the length of your backups using a particular tape drive, use this formula:

Actual data transfer rate = (Amount of data to back up)/((Number of drives) * (Tape drive transfer rate))

The following is an example of how to calculate the time required to back up to tape.

This example makes the following assumptions:

  • Amount of data to back up during a full backup = 1000 gigabytes (1 terabyte)

  • Daily backup window = 8 hours

  • Ideal transfer rate (data/(backup window)) =1000 gigabytes per 8 hours = 125 gigabytes per hour

Solution 1:

Tape drive = 1 drive, LTO-2

Tape drive transfer rate = 64.8 gigabytes per hour at 60% utilization with no compression, or 129.6 gigabytes per hour at 60% utilization with 2:1 compression

Actual data transfer rate, no compression = 1000 gigabytes/((1 drive) * (64.8 gigabytes per hour)) = 15.43 hours

Actual data transfer rate, 2:1 compression = 1000 gigabytes/((1 drive) * (129.6 gigabytes per hour)) = 7.72 hours

At 64.8 gigabytes per hour, an LTO-2 tape drive takes approximately 15.43 hours to perform a 1000-gigabyte backup. Under normal circumstances, the LTO-2 tape drive with no compression cannot perform the backup in eight hours. But, at 129.6 gigabytes per hour (with 2:1 compression), an LTO-2 tape drive takes 7.72 hours to perform a 1000-gigabyte backup.

In this example, you need to use compression or a faster tape drive, or add another LTO-2 tape drive.

Solution 2:

Tape drive = 1 drive, LTO-3

Tape drive transfer rate = 172.8 gigabytes per hour at 60% utilization with no compression, or 345.6 gigabytes per hour at 60% utilization with 2:1 compression

Actual data transfer rate, no compression = 1000 gigabytes/((1 drive) * (172.8 gigabytes per hour)) = 5.79 hours

Actual data transfer rate, 2:1 compression = 1000 gigabytes/((1 drive) * (345.6 gigabytes per hour)) = 2.89 hours

With a data transfer rate of either 172.8 or 345.6 gigabytes per hour, a single LTO-3 tape drive easily performs the 1000-gigabyte backup in less than 8 hours.

Depending on the factors that can influence the transfer rates of your tape drives, you can obtain higher or lower transfer rates. These example solutions are approximations of what you can expect.

Note also that a backup of encrypted data may take more time.

See Encryption and NetBackup performance

See Designing your backup system

See Faster tape drives are not always better.



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