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Can admins change the write back cache size?

Organizations may be surprised to find SSD space eaten up by write-back cache. Here's how to prepare for that.

The addition of write-back cache to Windows Server 2012 R2 doesn't directly affect the way that storage space is...

provisioned, but it does require additional awareness of cache storage demands.

Although solid-state drive capacities are growing and costs are falling, remember that each established 1 GB write-back cache will reduce the SSD's remaining storage capacity by 1 GB. For example, if you have a 100 GB SSD and create a new volume on magnetic disks along with a 1 GB write cache on the SSD, the SSD will only have 99 GB remaining. If 20 write-back caches are created to cover 20 virtual disk volumes, the same 100 GB SSD will only have 80 GB available. Organizations that make extensive use of write-back cache may discover a surprising amount of SSD space committed to cache use. This, in turn, may trigger additional and unexpected SSD purchases.

In order to optimize write-back caching on SSDs, administrators must be judicious about the way cache is used. First, it is not possible to change the write-back cache size: It's currently fixed at 1 GB for each associated storage volume (though this may change in the future). As a result, write-back cache should be used only for volumes created on slower magnetic disk -- there is no point creating an SSD cache for an SSD volume.

Reserve write caching for workloads that produce large, unpredictable levels of random writes. Not every workload needs a fast write cache, and applications that produce only infrequent writes will typically realize less benefit from write-back cache. So, the choice to use a write-back cache requires a clear knowledge of each application's read/write behavior. Administrators can usually employ simple monitoring tools like Performance Monitor, or PerfMon, in a test environment to understand each application's storage behavior over time.

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