How do I view and interpret all SSD Cache statistics?

You can view nominal statistics and detailed statistics for SSD Cache. Nominal statistics are a subset of the detailed statistics. The detailed statistics can be viewed only when you export all SSD statistics to a .csv file. As you review and interpret the statistics, keep in mind that some interpretations are derived by looking at a combination of statistics.

Nominal statistics

To view SSD Cache statistics, select Storage > Pools & Volume Groups. Select the SSD Cache that you want to view statistics for, and then select More > View Statistics. The nominal statistics are shown on the View SSD Cache Statistics dialog.
Note: This feature is not available on the EF600 storage system.

The following list includes nominal statistics, which are a subset of the detailed statistics.

Nominal statistic Description
Reads/Writes The total number of host reads from or host writes to the SSD Cache-enabled volumes.

Compare the Reads relative to Writes. The Reads need to be greater than the Writes for effective SSD Cache operation. The greater the ratio of Reads to Writes, the better the operation of the cache.

Cache Hits A count of the number of cache hits.
Cache Hits (%) Derived from Cache Hits / (reads + writes).

The Cache Hit percentage should be greater than 50 percent for effective SSD Cache operation. A small number could indicate several things:

  • The ratio of Reads to Writes is too small
  • Reads are not repeated
  • Cache capacity is too small
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 controller. Derived from allocated bytes / available bytes.

Cache Allocation percentage normally shows as 100 percent. If this number is less than 100 percent, 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 volumes, 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 percent, perhaps much lower. This number shows the percent of SSD Cache capacity that is filled with cache data. This number is lower than 100 percent because each allocation unit of the SSD Cache, the SSD 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.

Detailed statistics

The detailed statistics consist of the nominal statistics, plus additional statistics. These additional statistics are saved along with the nominal statistics, but unlike the nominal statistics, they do not display in the View SSD Cache Statistics dialog. You can view the detailed statistics only after exporting the statistics to a .csv file.

When viewing the .csv file, notice that the detailed statistics are listed after the nominal statistics:
Detailed statistics Description
Read Blocks The number of blocks in host reads.
Write Blocks The number of blocks in host writes.
Full Hit Blocks The number of blocks in 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 The number of host reads where at least one block, but not all blocks, were in the SSD Cache. A partial hit is an SSD Cache miss where the reads were satisfied from the base volume.
Partial Hits - Blocks The 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 hard disk drive (HDD) volume. 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, a different I/O characteristic type (file system, database, or web server) could improve the performance. It is expected that there will be a larger number of Partial Hits and Misses as compared to Cache Hits while the SSD Cache is warming.

Misses The number of host reads where none of the blocks were in the SSD Cache. An SSD Cache miss occurs when the reads were satisfied from the base volume. It is expected that there will be a larger number of Partial Hits and Misses as compared to Cache Hits while the SSD Cache is warming.
Misses - Blocks The number of blocks in Misses.
Populate Actions (Host Reads) The number of host reads where data was copied from the base volume to the SSD Cache.
Populate Actions (Host Reads) - Blocks The number of blocks in Populate Actions (Host Reads).
Populate Actions (Host Writes) The number of host writes where data was copied from the base volume to the SSD Cache.

The Populate Actions (Host Writes) count might be zero for the cache configuration settings that do not fill the cache as a result of a Write I/O operation.

Populate Actions (Host Writes) - Blocks The number of blocks in Populate Actions (Host Writes).
Invalidate Actions The number of times data was invalidated or removed from the SSD Cache. A cache invalidate operation is performed for each host write request, each host read request with Forced Unit Access (FUA), each verify request, and in some other circumstances.
Recycle Actions The number of times that the SSD Cache block has been re-used for another base volume and/or a different logical block addressing (LBA) range.

For effective cache operation, the number of recycles must be small compared to the combined number of read and write operations. If the number of Recycle Actions is close to the combined number of Reads and Writes, the SSD Cache is thrashing. Either the cache capacity needs to be increased or the workload is not favorable for use with SSD Cache.

Available Bytes The number of bytes available in the SSD Cache for use by this controller.
Allocated Bytes The number of bytes allocated from the SSD Cache by this controller. Bytes allocated from the SSD Cache might be empty or they might contain data from base volumes.
User Data Bytes The number of allocated bytes in the SSD Cache that contain data from base volumes.

The available bytes, allocated bytes, and user data bytes are used to compute the Cache Allocation percentage and the Cache Utilization percentage.