This article provides some recommendations to prevent the PVS Write Cache to fill up too fast. PVS Write Cache on RAM with OverFlow to HD improves the machines performance and it is the most recommended one. However, if the the HD dedicated to store Write Cache dumped from cache on RAM, this will affect the performance of the machines or they can even get hung. If the machines are Citrix VDAs, they eventually will unregister from the Delivery Controllers due to this problem. Besides, users will experience slowness or even lose their sessions. 

Instructions

1. Using ProcMon add filters "Path contains C:\" and "Operation is WriteFile" and apply them to find what processes are writing more frequently to C:\ drive (the vDisk on PVS target devices). This is the most common cause of this issue as all writes to the C:\ drive are sent to the Write Cache.
   • If there are processes found writing too frequently, then investigate the cause which could be different for each process depending what software belongs to.
   • To ensure accurate correlation between writes to the C: drive and growth of Write Cache File, make a copy of the Vdisk, set the RAM portion of Write cache to 0 and then use this disk for Procmon data collection.
2. If there is an Antivirus or Security Software installed, make sure that all Citrix recommended AV exclusions are implemented for all the Citrix components installed on the vDisk / Image. Refer to eDoc https://docs.citrix.com/en-us/tech-zone/build/tech-papers/antivirus-best-practices.html for more information.
   • Make sure that PVS Storage where the vDisks are located is excluded from Antivirus scanning on the PVS servers side, and on the PVS Target Devices side confirm that the Write Cache partition is also excluded from being scanned.
3. Check if vDisk is fragmented by mounting its base image file (.vhdx) on Windows (it can be done on the same PVS servers), then run any fragmentation trusted fragmentation analyzer. This is one of the causes of the Write Cache to fill up too fast, although is not the most common one.    
4. Configure the Write Cache for the vDisk on PVS console to at least 512 Mb for Desktop OS and 1024 Mb for Server OS. If a bigger value can be configured it will be even better as the more space available the longer will take to fill up.
5. If using Citrix or Microsoft roaming profiles, monitor the size of the profiles. As bigger is the user profile it will take longer to load it during the logon process and hence more writes will occur to C:\ drive causing the Write Cache on RAM to fill up faster.
   • If using Citrix UPM, make sure to configure the exclusions settings to prevent undesired / unneeded files to be synchronized to the profiles https://docs.citrix.com/en-us/profile-management/current-release/configure/include-and-exclude-items.html.
   • Also make sure to configure Folder Redirection policy settings are implemented to prevent that folders like the Desktop, Documents, Downloads etc are synchronized. Users tend to save big amount of files on these folders causing their profiles to grow over time https://docs.citrix.com/en-us/profile-management/current-release/configure/configure-folder-redirection.html
6. Make sure that Windows Updates Service is disabled and any other automatic update process from any other software / service. On PVS / MCS machines updates should be done manually
7. To analyze the PVS Write Cache with Overflow refer to Citrix eDoc  https://docs.citrix.com/en-us/tech-zone/build/tech-papers/digging-into-pvs-with-poolman-and-wpa.html which explains how Microsoft tool Windows Performance Analyzer can be used for this.
8. Make sure that any software is redirecting any type of data to the write cache disk attached to PVS Target machines. 

• https://www.citrix.com/blogs/2015/08/19/digging-into-pvs-with-poolmon-and-wpa/