PVS Internals 2 – How to properly size your memory

An article by Martin Zugec from Citrix Blogs

Summary

There are a few general recommendations that you should follow when doing memory sizing for PVS.

  • Do not use size of the vDisk to calculate memory requirements. The cache manager is caching blocks, not the whole files, so you don’t need to plan for caching the entire VHD in memory. You will probably be surprised how little memory you actually need for the average operating system.
  • Don’t aim to cache 100% of read operations. Don’t aim to provide 100% read operations from cache – you can probably provide 80% success rate with as much as 2-4GB of memory for each vDisk (with exceptions of course).
  • Don’t use standby cache for sizing. Windows will try to cache everything, as long as there is enough memory – and files are never removed from cache unless there is better use for that memory. If you will leave your PVS server for few weeks and then decide you need to buy more RAM because there is no free memory, you should return back to PVS Internals 1 and read it more carefully this time. As I’ve tried to explain, PVS actually requires a lot less memory than most people expect.
  • Don’t underestimate the memory of your VMs. Having a lot of memory for PVS server itself, while not having enough memory for your VMs is very common mistake. Don’t forget that your target devices are also using system cache – and this is reducing the re-reads from PVS server. You should reserve at least 500 MBs for single-user OS and at least 2GB for multi-user OS to be used by system cache.
  • Clear your cache before benchmarking. Make sure to always empty standby cache before recording any numbers to get clear results. Windows will try to cache everything, so it’s easy to get your cache “filled”. This is completely normal and as explained before, thanks to the prioritized cache manager, one-shot operations are not going to override your frequently accessed data blocks. For your final sizing numbers, don’t forget to add some overhead (typically around 15%).
  • Identify breakpoints during your tests. Don’t expect that Windows is fully loaded when the logon screen appears. As you will see in your tests, it is still reading data in the background.
  • Be aware of any component that reads all files. If you have incorrectly configured antivirus or other components that can read the disk, your standby cache can easily get filled. Make sure to monitor your cache usage to detect such issues in advance. This problem can occur on both PVS server and the target device itself.
  • Use our magic formula unless you have a very specific reason not to do so. This formula covers majority of implementations. Unless you have a very specific reason to do so or you have large number of vDisks, you should follow it. Just to remind you, here it goes again:

2GB + (#XA_vDisk * 4GB) + (#XD_vDisk * 2GB) + 15% (Buffer)

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