Product Documentation

Selecting the write cache destination for standard vDisk images

Sep 28, 2016

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

Considerations and requirements:

  • If 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.
  • If selecting cache on the target device RAM and Standard Image mode, the max size of the RAM write cache is determined by the registry setting WcMaxRamCacheMB in the BNIStack Parameters. This is a DWORD parameter. If the registry entry does not exist, then the default value used is 3584 MB.
  • Provisioning Services 7.7 only supports the use of Microsoft System Center Configuration Manager (ConfigMgr) Client as follows:
    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 list describe all valid write cache destination options.

Cache on device hard drive

Write cache can exist as a file in NTFS format, located on the target-device’s hard drive. This write cache option frees up the 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 vDisk mode is set to Private Image mode.

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 Console, right-click on the Provisioning Server, select Configure Bootstrap. On the General tab, click on the drop-down Bootstrap file option, then choose CTXBP.BIN. Citrix recommends that the local HDD (client side) drive has enough free space to store the entire vDisk.

Cache in device RAM

Write cache can exist as a temporary file in the target device’s RAM. This provides the fastest method of disk access since memory access is always faster than disk access.

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 will consume.

Compared to “Cache on device hard drive” cache mode, the VHDX block format has a faster file expansion rate. The local disk free space should be reconsidered to accommodate the streaming workload. To ensure target device reliability in high demand workload, Citrix recommends that local disk free space be larger than vDisk capacity size.

When the local disk is out of space, the target device vDisk IO goes in to a pause state waiting for more local disk free space to become available. This condition has a negative impact on workload continuity; thus, 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 vDisk IOs 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. In this configuration, all writes are handled by the Provisioning Server, which can increase disk IO and network traffic.

For additional security, the Provisioning Server can be configured to encrypt write cache files. Since the write-cache file does exist on the hard drive between reboots, the data will be 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 vDisk image. If a vDisk is set to Cache on server persistent, each target device that accesses the vDisk automatically has a device-specific, writable disk file created. Any changes made to the vDisk 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.

In order to restore a vDisk that uses Cache Persistent on Server, be sure to backup all vDisk files and associated user cache files prior to making any vDisk modifications.

The benefits of using this cache option include:
  • Saves target device specific changes that are made to the vDisk 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 vDisk force the cache file to be marked invalid. For example, if the vDisk 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 should be manually deleted.
Invalidating changes include:
  • Placing a vDisk in Maintenance
  • vDisk is placed in Private Image mode
  • Mapping the drive from the Console
  • Changing the location of the write cache file
  • Using Automatic update