Cloud Block Storage (CBS) Benchmarking

  • Last updated on: 2013-10-22
  • Authored by: Rackspace Support

by John Strunk

When considering benchmarking information for Cloud Block Storage, there are several things you need to consider to understand the performance differences, similarities, and how more than just throughput is needed to determine which volume is appropriate to your needs.

Throughput is not the same as Input/Output Operations Per Second (IOPS). Throughput does not show

  • random access times
  • access times
  • seek tests in general
  • latency
  • IOPS

Throughput is not the only method for choosing your disk strategy and making the appropriate choice.

The limitations of Cloud Servers using a 1 Gb link for CBS connectivity means that you’ll run into this bottleneck before limitations of the differences in the disk options SATA and SSD. This is why throughput and sequential reads and writes will be the same for the most part (1 Gb uplink is approximately 100 megabytes per second (MBps) with a 125 to 135 MBps theoretical maximum) on both of these disk types. This is shown by a quick test that gives the following results:

Volume              Sequential Reads          Sequential Writes

CBS-SATA          95981/sec                    16564/sec

CBS-SSD            95927/sec                    15633/sec

localVM disk      108463/sec                   1208/sec

Optimally, your block sizes should be 4K because this is the standard to which we gear all our tuning and configurations on the CBS Internet Small Computer System Interface (iSCSI) side. Block sizes of 4K, 8K and 16K are the most common and are less likely to saturate Cloud Server’s 1 Gb network link. You should not use 1 Mb, 4 Mb, and 1 G block sizes if database performance is your disk strategy. With those sizes, you’ll be saturating the 1 Gb network link long before reaching any IOPS limitations or performance differences between the volume types. There is probably not a scenario where you would want to use those large block sizes for performance, even on local disk, and would certainly not be the kind of numbers you’d use for database implementations. Remember that IOPS decrease as block sizes increase.

With correct block sizes - in this case 4K, you should see a performance difference of almost 4X using SSD, even with 1 Gb uplink limitations:

Volume              Random Seeks

CBS-SATA          473.4/sec

CBS-SSD           1969/sec

localVM disk       578.6/sec

Performance will get much better starting in IAD3 when Cloud Servers begins using 10 Gb uplinks to the back end. This will be a dramatic fundamental shift in the performance available from Cloud Servers to CBS volumes.

Continue the conversation in the Rackspace Community.