Storage Read Caching

Note: Storage Read Caching is available for XenServer Enterprise Edition customers, or those who have access to XenServer through their Citrix Virtual Apps and Desktops.

Read Caching improves a VM’s disk performance as, after the initial read from external disk, data is cached within the host’s free memory. It greatly improves performance in situations where many VMs are cloned off a single base VM for example, in Citrix Virtual Desktops Machine Creation Service (MCS) environments, as it will drastically reduce the number of blocks read from disk.

This performance improvement can be seen whenever data needs to be read from disk more than once, as it gets cached in memory. This is most noticeable in the degradation of service that occurs during heavy I/O situations. For example, when a significant number of end users boot up within a very narrow time frame (boot storm), or when a significant number of VMs are scheduled to run malware scans at the same time (anti-virus storms).

Note: The amount of memory assigned to the XenServer control domain (dom0) may need to be increased for the most efficient use of read caching. For instructions on increasing dom0 memory, see Changing the Control Domain Memory.

XenCenter displays the status of Read Caching on the VM’s General tab. Note that you cannot enable or disable Read Caching using XenCenter. For instructions on using Read Caching, refer to the XenServer Administrator’s Guide.

Read Caching is enabled by default, provided:

  • The XenServer host is licensed with a XenServer Enterprise Edition or a Citrix Virtual Apps and Desktops license.
  • The VM is attached to a VDI on a file-based SR such as NFS or EXT3. Note that Read Caching cannot be used with other SR types.
  • The VM is created from a fast clone or a snapshot, or the VM is attached to a read-only VDI.

For detailed information about Read Caching, refer to the XenServer Administrator’s Guide.

Storage Read Caching