Configuring Disks in Azure for More Performance

In an ongoing effort to get the most performance out of Azure, I’ve run several test that lead me to this next tip that is both very helpful and extremely easy implement. Remember, you’re paying for the time you rent the VM. So, the more you get out of each VM, the more you save.

How do I get more disk performance in Azure IaaS?

Basically, my¬†recommendation¬†comes down to this: Think of disks in Azure as a network resources, and not as a disk. That type of reasoning brought me to test out different NTFS cluster sizes (or “allocation units”).

As a side point – Exercise #4 of the “Windows Azure Platform Training Course” says

leave the default Allocation unit size

It doesn’t give a reason… and the reason is probably because it’s “the default”. However, I recommend that you max out the size to 64K (instead of the default 4K). In my (very expensive) testing, I see that this yields a 20% increase in disk performance. And that increase followed directly in to SQL Server disk performance as well!

As they say, a picture is worth a bunch of words – so here is what my testing revealed.

Explanation of the Performance Test

To explain the above – I mounted 16 disks to an Extra Large Azure IaaS VM. I then created a script that would stripe the disks together and format them NTFS with a 4K cluster size. After that, the script would write a 1GB file to the disk. The test was run 30 times, and the measurement was only measuring the time to write the data – not the time to create or delete the file.

Next, the script would reformat the striped volume as NTFS with a 64K cluster size. The 1GB-file-test would then commence.

This process was repeated many times – yielding 300 results for 4K and 300 results for 64K. Also, the method of going back and forth ruled out any thought of a particular time of day being the issue.

The average time it took to write the 1GB file with a 4K cluster size was 2.5 seconds. The average time using the 64K cluster size was 2.0 seconds. This is a 20% increase in disk performance. It was also interesting to note that the 4K cluster size suffered from “spikes” more often than the 64K size did. You can download the spreadsheet of data here:¬†AzureDiskHammer_log_20120827.csv

About Timothy Khouri

I'm a software architect who loves solving problems. I enjoy the bleeding edge, like the cutting edge and tolerate the dull edge of legacy systems.
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