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Key performance factors in Tape Offload performance
Note in the diagram below how the read performance from a D2D4312 (red line) increases with the number of
read streams – just like with backup.
If the D2D4312 reads with a single stream (to physical tape) the copy rate is about 370 GB/hour. However, if
the copy jobs are configured to use multiple readers and multiple writers then for example with four streams
being read it is possible to achieve 1.3TB/hour copy performance.
What this means in practice is that you must schedule specific time periods for tape offloads when the D2D
Backup System is not busy and use as many parallel copy streams (tape devices) as practical to improve the copy
performance.
Read performance graph
Much higher read throughput (for tape offload) with 4 streams
Single stream read
performance
Aggregate Throughput MB/sec
Read
Streams read/written concurrently
For example: if each month there is 15 TB of data to archive or offload from a D2D4312 to a 2-drive physical
tape library, then the copy speed will be about 200 MB/sec (720 GB/Hr).
To offload 15 TB in one go will take 21 hours and, depending on other activities happening on the D2D, it may
not be possible to spare a single 21 hour copy time to physical tape. In this case one option is to stagger the
monthly offloads to tape over separate weeks doing approximately 4 TB each week, and this would then occupy
just over 4 hours bandwidth per week from the D2D which is easier to schedule.
Summary of Best Practices
See Appendix C for a worked example using HP Data Protector.
1. The first recommendation is to really assess the need for tape offload: with a D2D replication copy is another
copy of the data really necessary? How frequently does the offload to tape really need to be? Monthly
offloads to tape are probably acceptable for most scenarios that must have offload to tape.