Dear Architect, is my write cache estimate correct?


The latest question into the Ask the Architect mailbag comes from Andy.  Andy is creating a Provisioning services design for an environment based on Windows Server 2008, with the write cache stored on a NetApp share.  Andy’s question is if the write cache estimates are correct.  Basically, Andy is estimating 650 MB write cache per virtual desktop.  He achieves this by taking the assigned RAM and multiplying it by 25%.

First, using Windows 2008 is great for Provisioning services as this provides the largest system cache for the vDisk, which will speed up delivery as local disk reads are not required as often.

Second, write cache is a tricky thing to determine. You best bet is to set this up and let users go at it for a few days to see what you end up with.  However, that might not be possible.  In that case ou have to remember that the write cache is based on a few things:

  1. Application delivery approach: Streamed apps will impact write cache more than installed apps, which impact the write cache more than hosted apps. I can tell you my streamed Office applications are consuming 300MB of space on my disk (which would mean 300MB of write cache if the application is not pre-cached).
  2. Reboot cycle: If the default behavior is to reboot the virtual desktop upon each logoff, this will keep the write cache small as it is deleted on each reboot.
  3. Pagefile: The pagefile is included within the write cache file.  I’m assuming this is the RAM portion of the formula.
  4. User work flow: What the user does will have an impact on the size.  Many of the apps require writes to the disk. The more apps a user utilizes, the greater the impact on the write cache.

That is just a summary of what is involved.  If you want to see the blackboard discussion, check out the Ask the Architect Write Cache Video.

What do you think? Did I miss anything? How are you estimating your write cache size as part of the design process?

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