High Write Latency - VM vs Host

[andvm]

Hi,

Am seeing high write latency (around 80ms) for a CentOS VM using the dd (writing at around 300 IOPS) command however when I check the Host latency it is around 3ms

Nothing else is writing to disk as this is a test VM I placed to specifically check the R/W latency. Read latency looked ok as it generated around 2K IOPS and VM perceived latency was around 3ms.

I took this values both from the Performance graphs of the Datastore for the Virtual Machines and the Host, and also from esxtop within the Virtual Machines and Disk views respectively.

Please note that the storage is an all flash fiber storage on not vSAN but thought I could maybe get some thoughts/feedback about why I see such big difference between the VM vs Host perceived latency.

[tayfundeger]

Hi,

Of course there will be such a difference. They vary depending on the hardware available on the virtual machine.

For example;

You can see them if the CPU on a virtual machine is insufficient.

The virtual machine's SCSI Controller Card causes these values to be different.

In short, the configuration of the virtual machine is important here. Therefore, there may be differences between virtual machine and host such as R / W, latency.

Thanks.

[andvm]

Indeed that makes sense, after all its just a basic CentOS install with no vmware tools etc..

Will check its resources during the run to see if I need to increase its specs.

Might use HCIBench as think it works for SAN also (used for vSAN in the past).

Thanks