VAAI and VM VDisks performance

[ejolaus]

Hi,

Apart from VM management operations such as cloning, migrating, creating vDisks, etc... should VAAI affect also the performance once the VM is created and configured?

In my case I am performing a test using Iometer within a VM with different configurations of thin/thick virtual disks, switching on and off VAAI and I realized that there is no impact on the results. I am wondering whether that is the expected result.

[chriswahl]

The basic three VAAI primitives handle:

• Atomic Test & Set (ATS), which is used during creation of files on the VMFS volume
• Clone Blocks/Full Copy/XCOPY, which is used to copy data
• Zero Blocks/Write Same, which is used to zero-out disk regions

Source

These are not typically triggered by operations within the guest VM.

[ejolaus]

Thanks for you answer. But in the case of the Write Same primitive, imagine that you are using a thin VDisk over a thin LUN and then you are using a write intensive application on the VM which uses that disk. Will that primitive be triggered since maybe most of the disk regions have to be zeroed before the writing operations?

[mcowger]

Yes, if the VM is thin and write intensive applications cause rapid expansion of the disk, then the primitive could be triggered.

[ejolaus]

Yes, thats what I thought but then, when performing the test it did not make any difference with VAAI activated.

I am using Iometer. Could it be because of the .tst file that Iometer creates which takes all the space on disk before starting the tests?

[mcowger · Accepted solution]

Its actually probably because the performance difference of a thin disk compared to an EZT disk is very very small (sub 2%), and so very difficult to measure.

[ejolaus]

EZT allocates and zeroes the disk at the beginning whereas thin provision neither allocates nor zeroes anything. Why does it not make any difference? My understanding is that, in a thin disk, everytime a new write operations occurs, it allocates the needed blocks and zeroes them before the write op. EZT does not do these operations because the disk is already allocated and zeroed. Is it right? In that case, it should have an impact on the performance.

[mcowger]

It does have an impact on performance - the ~2% I mentioned above.

With VAAI, the process is just efficient enough to not have a significant impact.

[ejolaus]

Ok. Thank you for your answers!