Dell PowerVault MD 32XX/36XX Series Storage Arrays CLI Guide

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Show SSD Cache Statistics

Description

This command displays data about the SSD cache usage.

Syntax

show ssdCache [ssdCacheName] ssdCacheStatistics [controller=(0|1|both)] [file="filename"]

Parameters

Parameter	Description
`ssdCache`	The alphanumeric identifier (including - and _) of the SSD cache for which you want to get information. Enclose the identifier in square brackets ([ ]). If the SSD cache name contains special characters or consists only of numbers, you also must enclose the identifier in double quotation marks (" ") inside square brackets.
`controller`	Each RAID controller module stores SSD cache metadata for the virtual disks that it owns. Therefore, the SSD cache statistics are maintained and displayed per RAID controller module. Valid RAID controller module identifiers are `0`, `1`, or both, where `0` is the RAID controller module in slot 0, `1` is the RAID controller module in slot 1, and both is both RAID controller modules. Enclose the RAID controller module identifier in square brackets ([ ]). If you do not specify a RAID controller module, the default value is `both`.
`file`	The file path and the file name to which you want to save the SSD cache statistics. Additional statistics are available when you save the statistics to a file. Enclose the file name in double quotation marks (" "). For example: file="C:\Program Files\CLI\logs \statistics.csv" .

Notes

The following statistics are displayed on the screen:

Reads	Total number of host reads of SSD cache-enabled virtual disks. 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.
Writes	Total number of host writes to SSD cache-enabled virtual disks.
Cache Hits	A count of the number of cache hits.
Cache Hits (%)	Derived from Cache Hits/total reads. The Cache Hit percentage should be greater than 50% for effective SSD cache operation. A small number could be indicative of several things: Ratio of Reads to Writes is too small Reads are not repeated Cache capacity is too small NOTE: To help determine the ideal SSD cache capacity, you can run the Performance Modeling Tool using the `start ssdCache [ssdCacheName] performanceModeling` command.
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.
Populate Actions (Host Reads) - Blocks	Number of blocks in Populate Actions (Host Reads).
Populate Actions (Host Writes)	Number of host writes where data was copied from the base virtual disk to the SSD cache. The Populate Actions (Host Writes) count may 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	Number of blocks in Populate Actions (Host Writes).
Invalidate Actions	Number of times data was invalidated/removed from the SSD cache. A cache invalidate operation is performed for every host write request, every host read request with Forced Unit Access (FUA), every verify request, and in some other circumstances.
Recycle Actions	Number of times that the SSD cache block has been reused for another base virtual disk and/or a different LBA range. For effective cache operation, it is important that the number of recycles is 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, then 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	Number of bytes available in the SSD cache for use by this RAID controller module. The available bytes, allocated bytes, and user data bytes are used to compute the Cache Allocation % and the Cache Utilization %.
Allocated Bytes	Number of bytes allocated from the SSD cache by this RAID controller module. Bytes allocated from the SSD cache may be empty or they may contain data from base virtual disks. The available bytes, allocated bytes, and user data bytes are used to compute the Cache Allocation % and the Cache Utilization %.
User Data Bytes	Number of allocated bytes in the SSD cache that contain data from base virtual disks. The available bytes, allocated bytes, and user data bytes are used to compute the Cache Allocation % and the Cache Utilization %.