Dell PowerVault MD3260i - Manual (Page 333)

Cache Allocation
(%) The amount of SSD cache storage that is allocated, expressed as a percentage of the SSD
cache storage that is available to this RAID controller module. Derived from allocated bytes /
available bytes.
Cache Allocation percentage normally shows as 100%. If this number is less than 100%, it
means either the cache has not been warmed or the SSD cache capacity is larger than all the
data being accessed. In the latter case, a smaller SSD cache capacity could provide the same
level of performance. Note that this does not indicate that cached data has been placed into
the SSD cache, it is simply a preparation step before data can be placed in the SSD cache.
Cache Utilization
(%) The amount of SSD cache storage that contains data from enabled virtual disks, expressed as
a percentage of SSD cache storage that is allocated. This value represents the utilization or
density of the SSD cache. Derived from user data bytes / allocated bytes.
Cache Utilization percentage normally is lower than 100%, perhaps much lower. This number
shows the percent of SSD cache capacity that is filled with cache data. The reason this
number is lower than 100% is that each allocation unit of the SSD cache, the SSD cache
cache-block, is divided into smaller units called sub-blocks, which are filled somewhat
independently. A higher number is generally better, but performance gains can be significant
even with a smaller number.
These additional statistics are included when you save the data to a file:
Read Blocks Number of blocks in host reads.
Write Blocks Number of blocks in host writes.
Full Hit Blocks Number of block cache hits.
The full hit blocks indicate the number of blocks that have been read entirely from SSD cache.
The SSD cache is only beneficial to performance for those operations that are full cache hits.
Partial Hits Number of host reads where at least one block, but not all blocks, were in the SSD cache. This
is an SSD cache miss where the reads were satisfied from the base virtual disk.
Partial cache hits and partial cache hit blocks result from an operation that has only a portion
of its data in the SSD cache. In this case, the operation must get the data from the cached HDD
virtual disk. The SSD cache offers no performance benefit for this type of hit. If the partial
cache hit blocks count is higher than the full cache hit blocks, it is possible that a different I/O
characteristic type (file system, database, or web server) could improve the performance.
Partial Hits -
Blocks Number of blocks in Partial Hits.
Partial cache hits and partial cache hit blocks result from an operation that has only a portion
of its data in the SSD cache. In this case, the operation must get the data from the cached HDD
virtual disk. The SSD cache offers no performance benefit for this type of hit. If the partial
cache hit blocks count is higher than the full cache hit blocks, it is possible that a different I/O
characteristic type (file system, database, or web server) could improve the performance.
Misses Number of host reads where none of the blocks were in the SSD cache. This is an SSD cache
miss where the reads were satisfied from the base virtual disk.
Misses - Blocks Number of blocks in Misses.
Populate Actions
(Host Reads) Number of host reads where data was copied from the base virtual.
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