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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-caching dictates the caching behavior:
5.1.3.1.1. Shared Desktop Mode
For shared desktops, the on-boot option is set to reset and the allow-caching flag 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-boot option is set to persist and the allow-caching flag 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>