Why is my disk I/O so slow when testing with DD?

We have found that the OS Template has a greater impact on DD test results that we expected. Some OSes will perform better than others for reasons unknown to us. If you find yourself with slow disk write speeds when testing with DD, try another OS and see if you experience the same results.

In our testing we have acheived the following results on a completely empty Xen node using the 64bit templates on the same VPS:

  • CentOS 5.6 = 50MB/s (3 tests)
  • Debian 5.0 = 71MB/s (3 tests)
  • Gentoo 2011.07 = 53MB/s (3 tests)
  • Slackware 13.37 = 31-45MB/s (5 tests)
  • Ubuntu 10.04 = 64-67MB/s (3 tests)
  • Ubuntu 11.04 = 49-56MB/s (3 tests)

As you can see some of the results are very different, but if you experience consistently low disk I/O speeds please open a ticket with your results and we will investigate further. Also keep in mind that DD is not the best method of testing disk I/O performance so if you can try using a benchmark test like hdparm it would be greatly appreciated.

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