VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Virtual SAN does not know if all of the data blocks in the cache line are dirty, or only
some of them. If the data block being read is not dirty, it is read from flash like we
saw earlier; if the data block being read is dirty/invalidated, it needs to be read from
the capacity layer. However, Virtual SAN cannot tell the difference.
So potentially the data block on a invalidated line can come from cache or it can
come from the magnetic disk. This is why it is called a “partial hit”. In this last
example, because we are attempting to read an invalid/dirty data block from read
cache, the block has to be fetched from the capacity layer.
In the 6.0 version of Virtual SAN, there is new read cache code which ensures that
there are no invalidated read cache lines that have their latest data on magnetic
disk. This means reads from invalidated cache lines will always get their data from
the flash.
Returning to the original graphs, there are still two that need explaining.
RC IOPS breakdown
This graphs basically details read cache I/O operations. This level of detail gives you
a pretty good idea of how read cache is being utilized on this host. The metrics
shown can be described as follows:
Total Reads – Total number of read cache reads per second
RC/Mem Reads – Number of read I/Os were served from the In-Memory
ReadCache
RC/SSD Reads – Number of read I/Os were served from the SSD ReadCache
Total RC hits – Total number of read cache hits
R-a-W-a-R – This is mostly for internal use. Normal workloads don’t have
this pattern, but unfortunately synthetic workload generating tools like
IOMeter does. This metric is to help detect such workloads. Workloads that
exhibit this behavior do not really benefiting from cache, because it is doing
writes after reads, invalidating cache lines.
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