Citrix Systems 6.1.0 manual VM Boot Behavior, Implementation Details and Troubleshooting

5.1.3. VM Boot Behavior

There are two options for the behavior of a VM VDI when the VM is booted:

1.Shared Desktop Mode

On VM boot, the VDI is reverted to the state it was in at the previous boot. All changes while the VM is running will be lost when the VM is next booted.

Select this option if you plan to deliver standardized desktops to which users cannot make permanent changes.

2.Private Desktop Mode

On VM boot, the VDI is in the state it was left in at the last shutdown.

Select this option if you plan to allow users to make permanent changes to their desktops.

5.1.3.1. VM Caching Behavior Settings

The VDI flag allow-cachingdictates the caching behavior:

5.1.3.1.1. Shared Desktop Mode

For shared desktops, the on-bootoption is set to reset and the allow-cachingflag is set to true, new VM data is written only to local storage – there will be no writes to shared storage. This means that the load on shared storage is significantly reduced. However the VM cannot be migrated between hosts.

5.1.3.1.2. Private Desktop Mode

For private desktops, the on-bootoption is set to persist and the allow-cachingflag is set to true, new VM data is written to both local and shared storage. Reads of cached data do not require I/O traffic to shared storage so the load on shared storage is somewhat reduced. VM Migration to another host is permitted and the local cache on the new host is populated as data is read.

5.1.4. Implementation Details and Troubleshooting

Q:Is IntelliCache compatible with XenMotion and High Availability?

A:You can use XenMotion and High Availability with IntelliCache when virtual desktops are in Private mode, that is when on-boot=persist

Warning:

A VM cannot be migrated if any of its VDIs have caching behavior flags set to on- boot=reset and allow-caching=true. Migration attempts for VMs with these properties will fail.

Q:Where does the local cache live on the local disk?

A:The cache lives in a Storage Repository (SR). Each host has a configuration parameter (called local-cache- sr) indicating which (local) SR is to be used for the cache files. Typically this will be a EXT type SR. When you run VMs with IntelliCache, you will see files inside the SR with names <uuid>.vhdcache. This is the cache file for the VDI with the given UUID. These files are not displayed in XenCenter – the only way of seeing them is by logging into dom0 and listing the contents of /var/run/sr-mount/<sr-uuid>

19