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Getting to know Dell PowerFlex v3.6.x

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Cache

PowerFlex offers a number of caching options, for the purpose of enhancing system performance.

The following caching options are supported by PowerFlex:

  • RAM Read Cache (using DRAM server memory)
  • Read Flash Cache (using SSD and NVMe SSD devices)
SSDs used for caching cannot be used for storage purposes.
NOTE:If CacheCade is used, you must configure External acceleration. Some devices are already benefitting from an external (For example, a non-PowerFlex related) acceleration/caching mechanism. This input is required in order to avoid alerts about media type mismatches. For example, when the user adds an HDD device which the SDS perceives as being too fast to fit the HDD criteria.

The following table summarizes information about the caching modes provided by the system.

Table 1. Caching modes
Mode Description Considerations Default Setting
RAM Read Cache (RMcache) Read-only caching performed by server RAM.

RAM Read cache, the fastest type of caching, uses RAM that is allocated for caching. Its size is limited to the amount of allocated RAM.

NOTE:

The amount that may be allocated is limited, and can never be the maximum available RAM.

Disabled, except when storage-only nodes are deployed.
Read Flash Cache (RFcache) Read-only caching performed by one or more dedicated SSD or NVMe SSD devices in the server.

RFcache uses the full capacity of SSD devices (up to eight) to provide a larger footprint of read-only LRU (Least Recently Used) based-caching resources for the SDS. This type of caching reacts quickly to workload changes to speed up HDD Read performance.

Several SSD devices can be allocated to a shared cache pool, and therefore the cache size is limited in size only by the amount of SSDs allocated for this purpose.

The RFcache driver must be installed during deployment. Caching devices can be defined either during the installation process or after deployment.

Limitations: The RFcache feature is only supported by 512 Byte devices for the cache use case. RFcache is not supported on 4K devices used for cache. However, RFcache acceleration of 4K storage devices is supported.

Windows backend is not supported.

Support matrix:

  • An RFcache device (flash device) can be partitioned only on Linux.
  • An SDS storage/source device cannot be partitioned if it needs to be accelerated by RFcache.
  • An SDS storage/source device as a file (over file system), cannot be accelerated by RFcache.
  • CloudLink Data at Rest Encryption (D@RE) is not supported on devices used for the RFcache feature.
Disabled
The following table illustrates the caching support matrix:
Table 2. Caching support matrix
System RFcache RMcache
PowerFlex Yes Yes
VxFlex Ready Node PowerEdge 13G servers Yes Yes
VxFlex Ready Node PowerEdge 14G servers Yes Yes

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