Database Reference
In-Depth Information
Having a cache block size that is too big can cause fragmentation in the cache and poor
utilization. This may cause a substantial portion of the cache resource to be unutilized
and therefore wasted. Figure 6.40 illustrates the impact of vFRC block fragmentation.
Figure 6.40 vFRC block fragmentation.
In Figure 6.40 , the vFRC block is set to a much larger size than the predominant IO size
—in this case, 128KB or 512KB versus the actual IO size of 8KB. As a result, a large
proportion of the blocks configured is wasted.
Tip
If in doubt about what your cache block size should be, start at 8KB. Having the
cache block size smaller than the actual IO size is better than having it oversized.
Your cache block size should evenly divide the predominant IO size to ensure
best performance and lowest latency. If your predominant IO size were 64KB,
then having a cache block size of 8KB or 16KB would be fine because it can
evenly divide the IO size.
The cache size and block size are manually set when you enable vFRC on a VM, and
they can be changed at runtime without disruption. Having the cache too small will
cause increased cache misses, and having it too big is not just wasteful, it will impact
your vMotion times. By default, when vFRC is configured, the cache of a VM will be
migrated when the VM is vMotioned. If it's set too big, this will increase the vMotion
times and network bandwidth requirements. You can, if desired, select the cache to be
dropped during a vMotion, but this will have an impact on SQL performance when the
VM reaches its destination while the cache is being populated again.
Caution
Make sure a large enough flash resource exists on each server in your vSphere
 
 
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