Database Reference
In-Depth Information
to note that this does not limit the capacity of open VMDKs on the ESXi 5.5 Host, just
how many of the pointer blocks can stay cached in RAM. If only 20% of all VMDK
blocks are active, you could conceivably be able to have 640TB or more of open
VMDK capacity on the host, while still having the active pointer blocks cached without
much, if any, performance penalty.
The way this new Pointer Block Eviction Process works gives you a sense of having an
almost unlimited amount of open VMDK capacity per ESXi 5.5 host. But it's not quite
unlimited; there is a tradeoff as the amount of active VMDK capacity on an ESXi 5.5
host increases. The tradeoff is possible Pointer Block Cache Thrashing, which may
impact performance.
With the default setting of MaxAddressableSpaceTB=32, the Pointer Block Eviction
Process won't kick in until the amount of open VMDKs exceeds 25.6TB. So if you
aren't expecting the VMs on your hosts to routinely exceed 25TB of open and active
VMDK blocks, there is probably no need to even look at adjusting
MaxAddressableSpaceTB; this saves you some host RAM that can be used for other
things. In most cases, you would only have to adjust MaxAddressableSpaceTB if the
active part of all open VMDKs on a host exceeds 25TB. If active VMDK blocks exceed
the capacity of the Pointer Block Cache, then thrashing could result from constantly
evicting and reloading pointer blocks, which may have a performance penalty.
You will see signs of Pointer Block Eviction in the VMKernel logs on your hosts if it is
occurring. Syslog, vCenter Log Insight, or Splunk will help you spot this type of
activity. If you start to notice any sort of performance impact, such as additional storage
latency visible in KAVG in ESXTOP, and a correlation to Pointer Block Eviction, then
that would be a sign you should consider adjusting MaxAddressableSpaceTB. If you're
planning to have 100TB of open VMDKs per host routinely, as in the case of large SQL
Servers, we recommend setting MaxAddressableSpaceTB = 64 and adjusting upwards
if necessary. If you're not concerned about the amount of RAM the Pointer Block Cache
will consume, you could consider setting it to the maximum of 128.
Increasing MaxAddressableSpaceTB may consume host RAM unnecessarily and so
should be considered along with the total RAM per host and the RAM that is likely to
be consumed by all VMs. 512MB of RAM consumed for Pointer Block Cache on a host
with 512GB of RAM or more is not significant enough to worry about, but could be
worth considering carefully if your hosts only have 32GB of RAM.
Caution
Any time you change an advanced parameter in vSphere, it's something that has to
be managed and considered when you are changing your environment. To “Keep
It Simple and Standardized” (Principle 5), you should avoid changing advanced
parameters if possible.
 
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