VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Some examples of random workloads include Online Transaction Processing (OLTP)
database access, such as Oracle. A common way of testing random I/O is using
DVDStore, which is an online e-commerce test application with a backend
database component, and a client program to generate workload.
Read versus write
We already mentioned that hybrid Virtual SAN configurations use flash for read
caching and write buffering. Sizing cache so that the workload can be largely
contained in cache will produce the most desirable performance results from Virtual
SAN. All of the writes will go to the write buffer in flash, but it is not guaranteed that
all of the reads will come from the read cache, also in flash. However, that is what
you would like to achieve, and avoid any read cache misses.
Read cache sizing is very important; write cache sizing somewhat less so.
Insufficient read cache causes cache misses, which are very expensive operations.
Writes are different; they always go to cache and are eventually destaged. More
write cache means that this destaging happens less often and more efficiently, but
the effect of having less write cache is much less dramatic when compared to read
cache.
In the current release, there is no control over the read/write cache ratios. It is
hard-coded at 70:30 write:read. However, there is the ability via the VM storage
policy to set a read cache reservation for your particular virtual machine, if you
think that it will benefit from having its own dedicated read cache, rather than
sharing it with other virtual machine storage objects. But do so carefully!
Where cache helps and where it may not
Flash as cache helps performance in two important ways. First, frequently read
blocks end up in cache, dramatically improving performance. Second, in hybrid
configurations, all writes are committed to cache first, before being efficiently
destaged to disks – again, dramatically improving performance.
However, data still has to move back and forth between disks and cache. A sustained
sequential write test ends up being a test of the back end disks, as does a sustained
sequential read test. We also mentioned that long periods of “burstiness” could fill
up the cache, and I/O may need to be satisfied from the magnetic disk layer.
Also, you need to be aware that most real-world application workloads take a while
for cache to “warm up” before achieving steady-state performance.
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