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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.

Next Steps

Why write back cache can create performance boosts

Making sense of write back cache requirements

This was last published in October 2014

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