End-users are complaining that an application running on an all-flash vSAN datastore is not performing well. Which performance chart should an administrator view to troubleshoot this issue?
A. Recovery Write IOPS
B. Resync Latency
C. Read Cache Read IOPS
D. Write Buffer Free Percentage
B
I believe that correct is D, I used the article below to justify my answer.
https://kb.vmware.com/s/article/2150012
SSD congestion is typically raised when the active working set of write IOs for a specific disk group is much larger than the size of the cache tier of the disk group. In both the hybrid and all-flash vSAN cluster, data is first written to the write cache (also known as write buffer). A process known as de-staging moves the data from the write buffer to the capacity disks. The write cache absorbs a high write rate, ensuring that the write performance does not get limited by the capacity disks. However, if a benchmark fills the write cache at a very fast rate, the de-staging process may not be able to keep pace with the arriving IO rate.
Also support answer c based on https://blogs.vmware.com/virtualblocks/2017/06/04/enhanced-resynchronization-metrics-vsan-6-6/
No solid reference found for B
Apologies. Wanted to say I also support answer B based on https://blogs.vmware.com/virtualblocks/2017/06/04/enhanced-resynchronization-metrics-vsan-6-6/
I agree.. It is definately not C. There is no read cache concept in All-Flash.
Another frustratingly vague question.
Definitely not C, since the cache on an all-flash is only used for writes.
D will tell you if your cache is destaging writes, but I would think you would need to look at other charts to determine whether destaging is having an impact.
I can’t find a definition of “recovery write IOPS”. Is it related to resync or UNMAP? I can’t tell.
Support for B:
https://kb.vmware.com/s/article/2150101
Of course, if your cluster were resyncing, it would either be due to a failure or a change you made. Either way, you would know it before going to performance charts.
I’m inclined to say D, given that “all-flash” was specifically called out.