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Difference between personal vDisk and Write cache drive in Citrix XenDesktop, using PVS?

Posted on 2014-01-26
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Last Modified: 2014-01-29
Hello there,

What is the difference between personal vDisk and Write cache drive in Citrix XenDesktop, when using Provisioning services.

For example do we need to have 2 x locally attached drives for each XenDesktop for write cache and Personal vDisk?

Please advise.

Thanks and Regards
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Question by:goprasad
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by:Carl Webster
ID: 39810852
Write Cache is where the difference stuff is kept and also where you can put persistent stuff like event logs, antivirus definition files and any other item(s) that need to persist between reboots of the desktop/vm.

PvD is where user personalization and user installed software goes.

You need to have two extra drives attached to your master image.

The write cache drive needs to be formatted and assigned a drive letter.

The PvD drive needs to be initialized but NOT formatted or assigned a drive letter.  The PvD drive gets handled by the PvD part of the VDA and is assigned a drive letter when you run the XenDesktop Setup Wizard.

I am writing an article on this very topic as we type.
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by:goprasad
ID: 39810898
WOW - Brilliant explanation.

Which means - as I am planning to use PVS to stream desktops, I will need to provision 2 x locally attached drives at hypervisor level,

Therefore:
 - If I assign drive letter D:\ for write cache - this needs to be formatted and can be thin provisioned.

- For personal vdisk - this gets handled when I run XenDesktop set-up wizard.

I was getting confused with following explanation of PVD : wherein I thought that write cache drive is provisioned within PVD.

By default, when you create a PvD, no matter the initial size, it will split the disk equally between User Profile and Applications.  E.g. if you have a 10GB PvD, 5GB will be for User Profile, Data, Docs, Personalisation etc. and 5GB for user installed applications.
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by:Carl Webster
ID: 39810912
In my testing, the Write Cache drive is always set as Drive D.  That is why I recommend moving the CD drive letter to like Z.

Create your Write Cache drive, format it and assign it Drive D.  That way your image (and future images) "know" there is a drive and since PVS/XD will make the drive D go ahead and do everything with it set as D.

I do not recommend doing anything as thin provisioned except in a lab.  You MUST make sure you have the space to handle all the space you have provisioned to be used so why not make everything think so you are not caught by a surprise during a peak usage time and everyone is breathing down your neck!!!

Yes, everything about PvD gets handled by the XD Setup Wizard and the PvD software.  If you are using XD7.1 make sure to update to PvD 7.1.1 BEFORE you do anything with it.

You are correct on the PvD split.
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Author Comment

by:goprasad
ID: 39810929
One last clarification: Is my calculation correct?

for a 300 user base for XenDesktops
- Master Image (base Win 7 32 bit OS plus core apps) - 25 GB
- Write Cache drive 25 GB x 300
- PVD 10 GB x 300

Therefore total disk space: 7500 + 3000 = 10.5 TB

Please advise.
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Carl Webster earned 500 total points
ID: 39810946
A 10GB write cache drive will be enough for a desktop that is rebooted once a week.  25GB is WAY overkill.

The PvD sizes I have worked with have been 20 to 30GB.  It all depends on whether you are using profile mgmt software or not, are you redirecting folders and how much software will users be installing.

There is one master image - 25GB.
300 10GB Write Caches drives is 3TB
300 10GB PvDs is 3TB

Total space is barely over 6TB.
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Expert Comment

by:Carl Webster
ID: 39810947
Oh, I forgot.  To make your life MUCH easier, do not have the Write Cache and PvD drives the same size.  So I would go with like a 12 or 15GB PvD.

Make sure you read this:

http://support.citrix.com/article/CTX131553
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Author Closing Comment

by:goprasad
ID: 39810952
Brilliant explanation of the terms Write Cache and PVD.
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by:Carl Webster
ID: 39817469
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