Note: This feature is only available in XenServer 7.1 and PVS 7.13 or later. 

PVS-Accelerator provides two cache modes:

• Memory only, in the Control Domain (Dom0) Memory. When selecting Memory only, the feature will use up to the specified cache size in the Dom0 memory. This option is only available after additional memory has been assigned to the Control Domain.
• Memory and Disk, on a Storage Repository (SR). When selecting a SR, the feature will use up to the specified cache size on the SR. It will also implicitly use available Control Domain memory as a best effort cache tier.

• If neither Memory only nor a SR is specified, the read cache will not be activated.
• PVS-Accelerator has been designed to utilize either memory only, or a combination of disk and memory. Irrespective of the configuration choice, customers should increase the amount of memory allocated to the Control Domain to ensure there is no system performance degradation.

• The tmpfs SR is leveraging Dom0 memory using tmpfs
• The contents of a tmpfs SR are wiped implicitly on host reboot
• Explicitly uses available Dom0 memory

• Using memory means consistently fast Read/Write performance when accessing or populating the cache.

• Hardware must be sized appropriately as the RAM used for cache storage will not be available for VMs.
• Control Domain memory must be extended before configuring cache storage.

• The VDI is attached to Dom0
• A XFS file system is nested into the VDI
• Memory-caching is implemented implicitly using the Linux page cache by doing reads and writes without O_DIRECT on the nested file system. Available Dom0 memory is used transparently. 

• Most recently read data will be cached in the memory on a best effort basis, so accessing the data can be as fast as using the Control Domain memory.
• The cache can be much larger if it is located on an SR. The cost of the SR space is typically a fraction of the cost of the memory space. This means, caching on an SR can take more load off the PVS Server.
• Control Domain memory setting does not have to be modified. The cache will automatically use the memory available in the Control Domain and never cause the Control Domain to run out of memory. 

• If the SR is slow and the requested data is not in the memory tier, the caching process can be slower than a remote PVS Server.

1. Citrix recommends that you allocate at least 4GB of Control Domain memory per host to avoid frequent disk accesses that cause higher read-latency and consequently degrade performance.
2. Citrix recommends that you allocate at least 5GB of cache space per vDisk version that will be actively used on.