Monday, January 11, 2010

VDI: User Data Redirection

In order to take full advantage of the advanced image management capabilities in a virtual desktop environment you have to homogenize the image by ensuring user data is redirected not written to the desktop image. Within a windows environment user configuration information is typically stored in local or roaming profiles. In order to ensure this information is maintained across multiple sessions on different desktops the profile is typically stored centrally and cached locally at login (roaming). When the user logs off, any changes are synchronized to the centrally stored profile. This technology has been around for many years and has been used in both desktop and terminal server environments. Profiles can be configured as read-only (mandatory), read-write (normal), or a mixture of both (flex) profiles. Flex profiles are based on a mandatory profile but user changes are written to a separate location such as a user directory. The Flex profile merges both the read-only profile and user customizations to provide the speed of a mandatory profile while still allowing user customizations. A number of things must work in harmony in order to ensure a user profile loads and unloads properly; the profile directory must be available, the user must have the appropriate permissions, adequate space must be available on the login device and all this must happen within a reasonable window of time to not affect the user. The same mixture of technology must also work when the user logs off to ensure that any changes are properly captured and the profile unloads cleanly. If a user is logged into two separate environments the profile that is unloaded last will overwrite any prior updates. Given the number of components that must interoperate it is quite common to experience many operational challenges when introducing roaming profiles.


1. User data needs to be redirected from the desktop image

VMware and Citrix approach the challenge of redirecting user data differently. VMware uses a separate user data disk that is thinly provisioned to a vmdk (Virtual Machine Disk) file. This redirection is accomplished through a local policy that is applied when you install the agent software on the virtual machine. Essentially VMware is using a local profile redirected or stored on a separate disk partition. The benefit to this approach is that you have the performance of a local profile with none of the draw backs of a roaming profile stored on a file server. The drawback of this approach is the flexibility that is lost by not having a centrally stored profile. A roaming profile can be universally applied to both a virtual desktop and terminal services environment vs. having to manage separate profiles depending on what the user is connecting to (desktop, shared server environment). Citrix provides a profile optimization service that ensures that profile changes are merged not discarded based on the profile or session that is exited last or essentially a variation of Flex profiles. The Citrix solution was integrated into their product line after sepagoProfile software was acquired from Sepago. The benefit to this approach is that the profile can be universally applied between either a virtual desktop or XenApp environment provided the operating systems are compatible (i.e. WinXP and Windows 2003, Vista and Windows 2008)

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