Storage Read Caching


Storage Read Caching is available for Citrix Hypervisor Premium Edition customers, or those customers who have access to Citrix Hypervisor through their Citrix Virtual Apps and Desktops entitlement.

Read Caching improves a VM’s disk performance as, after the initial read from an 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 as it drastically reduces the number of blocks read from disk. For example, Read Caching improves performance in Citrix Virtual Desktops Machine Creation Service (MCS) environments.

This performance improvement can be seen whenever data must be read from disk more than once, as it gets cached in memory. This performance difference 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 narrow time frame (boot storm)
  • When a significant number of VMs are scheduled to run malware scans at the same time (antivirus storm)


The amount of memory assigned to the Citrix Hypervisor control domain (dom0) might 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. You cannot enable or disable Read Caching using XenCenter. For instructions on using Read Caching, see Storage read caching.

Read Caching is enabled by default, provided:

  • The Citrix Hypervisor host is licensed with Citrix Hypervisor Premium 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/EXT4. 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, see Storage read caching.

Storage Read Caching