PCS: Determining If PAM Will Improve Performance
To begin using PCS, you enable the feature with the command:
options flexscale.enable pcs
options flexscale.enable off
With PCS enabled, you can find out what's happening using the following command:
stats show -p flexscale-pcs
Sample output is shown below
Use the following guidelines to help you interpret the data:
· If the hit/(invalidate+evict) ratio is small, then a lot of data is being discarded before it is used. The instance (ec0, ec1, ec2) may be too small.
· If the (hit+miss)/invalidate ratio is too small, it might indicate a workload with a large amount of updates; switch to metadata mode and check the hit% again.
· If the usage is stable and there are a small number of invalidates and evictions, then the working set fits well.
· The KB/s served by the cache is approximately equal to the hit/s × 4KB per block.
Note that the three caches simulated in PCS are cascading caches. In the example above, ec0 represents the first cache of size 8GB, ec1 represents the second cache of size 8GB, and ec3 represents the third cache of size 16GB. The hit per second for a 32GB cache is the sum of all the hits per second for all three caches. The key advantage of cascading caches is that in the process of measuring an accurate hit rate for a 32GB cache, we also obtain hit rate estimates of both 8GB and 16GB caches. This gives us three points on the hit rate curve and the ability to estimate hit rates for intermediate cache sizes.
PAM and FlexShare
FlexShare™ is a Data ONTAP option that allows you to set priorities for system resources (processors, memory, and I/O) at a volume level, thereby allocating more resources to workloads on particular volumes when the controller is under significant load. FlexShare is fully compatible with PAM, and settings made in FlexShare apply to the data kept in the PAM cache. With FlexShare, finer-grained control can be applied on top of the global policies you implement with PAM. For example, if an individual volume is given a higher priority with FlexShare, data from that volume will receive a higher priority in the cache.