VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
This view introduces two new graphs, Evictions and WriteBuffer Fill. Both of these
are references to the caching layer/SSD. Since Virtual SAN configurations use a
write buffer for I/O accelerations, these graphs give insight into how the write
buffer is behaving.
WriteBuffer Fill
This metric tells us how much the write buffer is being consumed. One would expect
that on a reasonably balanced system that a significant amount of write buffer is
consumed. This is a good metric to check if you are planning to place more
workloads on Virtual SAN. It can tell you whether you have enough flash capacity to
cater for the additional workloads.
Evictions
This metric tells us how often Virtual SAN has to evict data blocks from the write
buffer to make room for new data blocks. In an optomized system, we would expect
the working set of an application running in a virtual machine on Virtual SAN to
mostly reside fully in cache. Therefore we should not expect to see too many
evictions on an optomized Virtual SAN system. Excessive evictions could mean that
there are workloads running that are not suitable for a caching storage system like
Virtual SAN (sustained sequential write operations), or that the flash cache has been
undersized for the workload requirements.
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