Estimating the effect of multiple copies on backup performance

The Multiple copies option of the Schedule attributes takes one stream of data that the bptm buffers receive and writes to two or more destinations sequentially. (Previous releases of NetBackup referred to this option as Inline Copy, Inline Tape Copy, or ITC.) The time to write to multiple devices is the same as the time required to write to one device multiplied by the number of devices. The overall write speed, therefore, is the write speed of a single device divided by the number of devices.

The write speed of a backup device is usually faster than the read speed of the source data. Therefore, switching to multiple copies may not necessarily slow down the backup. The important figure is the write speed of the backup device: the native speed of the device multiplied by the compression ratio of the device hardware compression. For tape backups, this compression ratio can be approximated by looking at how much data is held on a single tape (as reported by NetBackup). Compare that amount of data with the uncompressed capacity of a cartridge.

For example:

LTO generations 6, 7, and 8 tape drives use a 2.5:1 compression ratio compared to the earlier LTO generations' 1.5:1 and 2:1 compression ratios. An LTO gen 6 drive has a native write speed of 160MB/s using the 2.5:1 compression ratio has a native write capacity of 160MB/s and a compression ratio of 400MB/s. Thus, 160MB/s * 2.5 = 400MB/s.

An LTO gen 6 cartridge has an uncompressed capacity of 2.5TB. The native write speed is 160MB/s. Using the 2.5:1 compression ratio, the compressed capacity will be 2.5TB * 2.5 = 6.25TB and a compressed write speed of 160MB/s * 2.5 = 400MB/s. Newer generations of LTO will follow this format.

If multiple copies to two LTO gen 6 drives are enabled, the overall write speed is 160/2 = 80MB/s.

If the backup normally runs at 80MB/s (the read speed of the source data is 80MB/s), multiple copies do not affect the backup speed. If the backup normally runs at 160MB/s, multiple copies reduce the speed of the backup.