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VNX5200. Why Fast Cache isn't used?
Hi.
We have VNX5200, 2 pools was created.
pools configurations:
first pool - 5 SAS Fast VP, 9 SAS, 8 NL SAS;
second pool - 9 SAS, 8 NL SAS.
FAST Cache is also created and enabled on both pools.
Both pools are used for storage of the virtual machines.
On the first pool about 50 VM's, on the second 10.
I watch performance counters in Unisphere Central and I see that, in some time cache hit ration is equal 99,9%
But Fast Cache cache hit ration is always equal 0.
аnd Fast Cache Percent Dirty Pages (in the Unisphere) is also equal 0.
Why Fast Cache isn't used?
PS. Sorry, for my English.
kelleg
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March 2nd, 2015 13:00
1. FAST Cache Hit Ratio is a value on the Pool - each Pool keeps a separate set of data for the FAST cache for its own Pool
2. FAST Cache Dirty Pages - VNX2 uses a different FAST Cache method than the old VNX arrays. On the VNX MCx the FAST Cache acts more like a Write-Through cache than a Write-back cache. When you write to data contained in the FAST Cache, that changes the state of the data from "Clean" to "Dirty". Instead of keeping the "Dirty" data in FAST cache, the data is flushed to the disks very quickly, which makes the data in FAST Cache change from Dirty to Clear, but the data is still in FAST Cache waiting for more hits. You would see Dirty Pages spike up then disappear. This is the normal operation. This is also explained in the "VNX-MCx-Multicore-Everything.pdf" on page 29 in the "Page Management" section. You can find this document on emc.support.com.
https://support.emc.com/docu48786_White-Paper:-VNX-MCx-Multicore-Everything.pdf
glen