Selecting the write cache destination for standard virtual disk images
Citrix Provisioning supports several write cache destination options. However, the recommended option is cache in device RAM with overflow on the hard disk.
If migrating from older local hard disk caches to cache in device RAM with overflow to hard disk, you must reevaluate your local disk cache size. This is because the new RAM cache with overflow to hard disk uses a larger segment size and grows faster and larger. For more detailed information about how the RAM cache with overflow functions, see Size Matters: PVS RAM Cache Overflow Sizing.
The write cache destination for a virtual disk is selected on the General tab, which is available from the vDisk File Properties dialog.
The following sections describe all write cache destination options.
Cache in device RAM
Write cache can exist as a temporary file in the target device’s RAM. This functionality provides the fastest method of disk access since memory access is always faster than disk access. The maximum RAM write cache size is determined by the registry setting
- The target device becomes unstable and can crash if the target device’s RAM write cache is full.
- For Windows 10 version 1803, the functionality cache in device RAM is not supported. A target device crashes when it fails to use reserved memory from bootstrap. Citrix recommends using Cache in device RAM with overflow on hard disk. This issue applies to legacy bootstrap, it does not apply to UEFI bootstrap configurations.
Cache in device RAM with overflow on hard disk
This write cache method uses VHDX differencing format:
- When RAM is zero, the target device write cache is only written to the local disk.
- When RAM is not zero, the target device write cache is written to RAM first. When RAM is full, the least recently used block of data is written to the local differencing disk to accommodate newer data on RAM. The amount of RAM specified is the non-paged kernel memory that the target device consumes. Compared to Cache on device hard drive cache mode, the VHDX block format has a faster file expansion rate.
When the local disk is out of space, the target device virtual disk I/O goes in to a pause state. It waits for more local disk free space to become available. This condition has a negative impact on workload continuity. Citrix recommends allocating enough local disk free space.
The amount of RAM specified does not change the local disk free space requirement. The more RAM assigned, the more virtual disk I/Os temporarily saved in RAM cache before data gets flushed back to the VHDX file. The RAM reduces the initial VHDX expansion rate.
The registry setting
WcMaxRamCacheMBis not used when configuring the Cache in device RAM with hard disk overflow. When using this write cache mode on the provisioning management console, the value specified from the maximum allocated size is used.
For more information on RAM cache overflow sizing, see Size Matters: PVS RAM Cache Overflow Sizing.
Cache on a server
Write cache can exist as a temporary file on a provisioning server. The server handles all writes, which increases disk I/O on the server and network traffic. For that reason, this mode is not recommended.
For extra security, the server can be configured to encrypt write cache files. Since the write-cache file does exist on the hard drive between reboots, the data is encrypted in the event a hard drive is stolen.
Consider the performance impact of using server side caching. This consideration applies to both persistent and non-persistent cache.