Selecting the write cache destination for standard virtual disk images

Citrix Provisioning supports several write cache destination options. The write cache destination for a virtual disk is selected on the General tab, which is available from the vDisk File Properties dialog.

Considerations and requirements:

  • Consider the impact of using server side persistent write cache. Only use persistent cache where unauthorized users have unprivileged access to a machine. Ensure that machines are not shared among users.
  • If you are selecting cache on local hard drive, ensure that the hard-disk drive is formatted with NTFS for Window devices, with a minimum of 500 MB.
  • When using cache on the target device RAM in standard image mode, the max size of the RAM write cache is set by the registry setting WcMaxRamCacheMB. This registry setting appears in the BNIStack parameters. This registry setting is a DWORD parameter. If the registry entry does not exist, then the default value used is 3584 MB.
  • Support for the Microsoft System Center Configuration Manager (ConfigMgr) Client is:
ConfigMgr Client Cache on device hard drive Cache in device RAM with overflow on hard disk Cache in device RAM
ConfigMgr 2007 - all not supported not supported not supported
ConfigMgr 2012 supported supported not supported
ConfigMgr 2012 SP1 supported supported not supported
ConfigMgr 2012 R2 supported supported not supported
ConfigMgr Client Cache on server Cache on server persisted Cache on device hard drive persisted
ConfigMgr 2007 - all not supported not supported not supported
ConfigMgr 2012 not supported not supported not supported
ConfigMgr 2012 SP1 not supported not supported not supported
ConfigMgr 2012 R2 not supported not supported not supported

The following sections describe all valid write cache destination options.

Note:

Provisioning Services version 7.12 introduced Linux streaming. When using this feature, consider that caching options on a Linux target device are the same as on a Windows device. For more information about Linux streaming, see the installation article.

Cache on device hard drive

Write cache exists as a file in NTFS format on the target-device’s hard drive. This write cache option frees up the Citrix Provisioning server since it does not have to process write requests and does not have the finite limitation of RAM.

The hard drive does not require any additional software to enable this feature.

Note:

The write cache file is temporary unless the virtual disk mode is set to Private Image mode.

Important: The virtual disk cache type field Cache on device hard drive is deprecated and will be removed in a future release. Citrix recommends using one of the other available cache types. For more information, see the Deprecation article.

Cache on device hard drive persisted (experimental phase only)

The same as cache on device hard drive, except cache persists. This write cache method is an experimental feature and is supported only for NT6.1 or later. This method also requires a different bootstrap. To select the correct bootstrap from the Citrix Provisioning console, right-click on the provisioning server, select Configure Bootstrap. On the General tab, click the menu Bootstrap file option, then choose CTXBP.BIN. Citrix recommends that the local HDD (client side) drive has enough free space to store the entire virtual disk.

Important

The virtual disk cache type field Cache on hard drive persisted is deprecated and will be removed in a future release. Citrix recommends using one of the other available cache types. For more information, see the Deprecation article.

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.

Tip

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. The local disk free space is reconsidered to accommodate the streaming workload. To ensure target device reliability in high demand workload, Citrix recommends that local disk free space is larger than virtual disk capacity size.

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 all data gets flushed back to the VHDX file. The RAM reduces the initial VHDX expansion rate.

Cache on a server

Write cache can exist as a temporary file on a provisioning server. The server handles all writes, which can increase disk I/O and network traffic.

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.

Cache on server persistent

This cache option allows for the saving of changes between reboots. Using this option, after rebooting, a target device is able to retrieve changes made from previous sessions that differ from the read only virtual disk image. If a virtual disk is set to Cache on server persistent, each target device that accesses the virtual disk automatically has a device-specific, writable disk file created. Any changes made to the virtual disk image are written to that file, which is not automatically deleted upon shutdown.

The file name uniquely identifies the target device by including the target device’s MAC address and disk identifier. A target device can be assigned to multiple vDisks and therefore have multiple cache files associated to it.

To restore a virtual disk that uses Cache Persistent on Server, be sure to back up all virtual disk files and associated user cache files.

The benefits of using this cache option include:

  • Saves target device specific changes that are made to the virtual disk image.
  • Same benefits as Standard Image Mode.

The drawbacks of using this cache option include:

  • The cache file is available so long as the file remains valid. Any changes made to the virtual disk force the cache file to be marked invalid. For example, if the virtual disk is set to Private Image Mode, all associated cache files are marked invalid.

Note:

Cache files that are marked as invalid are not deleted. Periodically, these files are manually deleted.

Invalidating changes include:

  • Placing a virtual disk in maintenance
  • Virtual disk is placed in private image mode
  • Mapping the drive from the console
  • Changing the location of the write cache file
  • Using automatic update

Tip:

Consider the impact of using server side persistent write cache. When administering this functionality, persistent cache is only used where unauthorized users have unprivileged access to a machine. Ensure that machines are not shared among users.