Zeroedthick and Thin Virtual Disks Zero-On-New-Write Performance

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In addition to accelerating up eagerzeroedthick deployment, WRITE SAME also improves performance within thin and zeroedthick virtual disks. Since both types of virtual disks zero-out blocks only upon demand (new writes to previously unallocated blocks) these new writes suffer from additional latency when compared to over-writes. The introduction of WRITE SAME reduces this latency by speeding up the process of initializing this space.

The following test was created to ensure that a large proportion of the workload was new writes so that the write workload always encountered the allocation penalty from pre-zeroing (with the exception of the eagerzeroedthick test which was more or less a control). Five separate tests were run:

  1. Thin virtual disk with WRITE SAME disabled.
  2. Thin virtual disk with WRITE SAME enabled.
  3. Zeroedthick virtual disk with WRITE SAME disabled.
  4. Zeroedthick virtual disk with WRITE SAME enabled.
  5. Eagerzeroedthick virtual disk

The workload was a 100% sequential 32 KB write profile in all tests. As expected the lowest performance (lowest throughput, lowest IOPS and highest latency) was with thin or zeroedthick with WRITE SAME disabled (zeroedthick slightly out-performed thin). Enabling WRITE SAME improved both, but eagerzeroedthick virtual disks out-performed all of the other virtual disks regardless of WRITE SAME use. With WRITE SAME enabled eagerzeroedthick performed better than thin and zeroedthick by 30% and 20% respectively in both IOPS and throughput, and improved latency from both by 17%.

The following three charts show the results for throughput, IOPS and latency.

Figure 17. IOPS, Throughput and Latency differences across virtual disk types (Y-axis in miliseconds)

Note that all of the charts do not start the vertical axis at zero—this is to better illustrate the deltas between the different tests.

Note:

It is important to understand that these tests are not meant to authoritatively describe performance differences between virtual disks types—instead they are meant to express the performance improvement introduced by WRITE SAME for writes to uninitialized blocks. Once blocks have been written to, the performance difference between the various virtual disk types evaporates. Furthermore, as workloads become more random and/or more read intensive, this overall performance difference will become less perceptible. This section is mostly a lab exercise, except for the most performance-sensitive workloads, performance should not be a huge factor in virtual disk type choice.

From this set of tests we can conclude:

  1. Regardless of WRITE SAME status, eagerzeroedthick virtual disks will always out-perform the other types for new writes.
  2. The latency overhead of zeroing-on-demand with WRITE SAME disabled is about 30% (in other words the new write latency of thin/zeroedthick is 30% greater than with eagerzeroedthick).
    1. The latency overhead is reduced from 30% to 20% when WRITE SAME is enabled. The duration of the latency overhead is also reduced when WRITE SAME is enabled.
  3. The IOPS and throughput reduction caused by zeroing-on-demand with WRITE SAME disabled is about 23% (in other words the possible IOPS/throughput of thin/zeroedthick to new blocks is 23% lower than with eagerzeroedthick).
    1. The possible IOPS/throughput reduction to new blocks is reduced from 23% to 17% when WRITE SAME is enabled.