When trying to decide between storage tiering or a storage cache in virtual desktop infrastructure (VDI) environments,...
the answer depends on the VDI platform, type of hardware used and how your virtual machines are configured.
Although alternate methods are available, storage tiering and caching are commonly based around the use of flash storage. There might, for instance, be one or more solid-state drives (SSDs) that are set aside for tiering or caching purposes.
In the case of tiering, the SSD acts as the high-speed tier. Each vendor has its own way of doing things, but tiering generally benefits read operations. The system monitors storage read requests to determine which storage blocks are read most frequently. Those blocks, sometimes referred to as hot blocks, are dynamically moved to the high-speed tier so they can be read as efficiently as possible. As some of the hot blocks begin to cool off, those blocks may be dynamically migrated back to the standard tier to make room for hotter blocks on the high-speed tier.
In contrast, a storage cache generally benefits write operations. There is a limit to the speed with which data can be written to rotational media. As such, space may be reserved on flash storage for use as a cache buffer. Write operations are written to the high-speed cache and later committed to the standard storage when I/O loads are light enough that the standard storage can efficiently handle the incoming data.
In a VDI environment, storage tiering tends to be more beneficial than storage caching because virtual desktops are read-intensive. Typically, most of the write operations are directed elsewhere, such as to a user's home directory stored on a network share.
There tends to be a lot of redundancy in VDI environments because each virtual desktop likely uses the same system files. As such, system files that are frequently read on one virtual desktop will probably be frequently accessed on others. Storage tiers would therefore tend to benefit all the virtual desktops and improve performance across the board.
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