Understanding Backup Performance
Performance of Alike backups has various facets, which can broadly be broken down into several areas:
1) Data read time
2) Data write time
3) Commit time and other time
Read time is the amount of time the A2 must spend reading data off your virtual disks. If you have CBT enabled, subsequent runs will skip most unchanged areas of the disk. Since most VMs don't change very much between backups, this is a considerable optimization. Obviously, for VMs that change a lot, this optimization is slightly less impactful. When troubleshooting data read time issues, you probably want to benchmark your underlying VM disk storage. You can get a rough-and-ready idea by running an ABD diagnostic job if you are running Xen. But this will not test all your SRs, so if you have many SRs, this test will not be sufficient.
Data write time is the amount of time the A2 must spend writing data to the ADS. Data is sent from the backup agent to your A2 over a socket. The A2 then further processes this data and sends it to the ADS. A very significant bottleneck here will eventually result in socket timeouts and backup failures. It's less common that write times are a gating factor on smaller A2 installs, and more typically become a factor for scaling up larger deployments where many VMs are all being backed up simultaneously to the same A2.
You can simply watch the dashboard to see how the A2 is consuming network and storage resources. In general, the gating factor in most cases is the ADS storage. Faster ADS disk that can buffer and handle your A2 block size effectively is critical.
Commit and other time. In general, this area of the A2 is rarely a performance bottleneck for backup times. But for larger A2 deployments, it still bears keeping in mind. Making sure you have fast local disks for your A2 system disk will help with commit, purge, and other maintenance operations. As all I/O ultimately contends for priority on your A2, addressing local disk performance will help you scale. As will choosing an appropriate block size for your needs.