This is pretty cool stuff!
One of the most welcome new features in Windows Server 2012 R2 is the ability to create native tiered storage. In case you are unfamiliar with the feature, a storage pool can contain a mixture of traditional hard disks and solid-state disks. When a virtual hard disk is created on top of the storage pool, the administrator is given the option of providing the new virtual hard disk with tiered storage. If the administrator chooses to enable this option then the most frequently read blocks (which are known as hot blocks) will be automatically copied to solid state storage so that they can be accessed with the greatest possible efficiency.
When you create a virtual hard disk with tiered storage, Windows allows you to specify the amount of traditional hard-disk space and the amount of solid-state storage space that you want to use for the new virtual hard disk. What you might not realize however, is that your fast storage tier might not end up being quite as large as you specify.
If you create a fast tier that is more than 1 GB in size Windows will use 1 GB of SSD space as a write-back cache. This write back cache improves performance by intercepting and smoothing out write operations. The disadvantage, of course, is that the fast storage tier is actually 1 GB smaller than you specify because of the space lost to the write-back cache.
With that being the case, it is a good idea to account for the lost space up front by making your fast storage tier 1 GB larger than you otherwise would. Of course this raises another question. Is it possible to disable or resize the write-back cache?
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