There are two multipathing drivers that have been available since XenServer 5.0 SP2.  They are the DMP (dynamic multipath) and MPP (multipath proxy).  DMP is the one that is used by default.  In fact when you go into XenCenter and enable the check box for ‘multipathing’ you are essentially turning on DMP.  Normally this works well but the IBM DS4800 and other storage controllers that use RDAC (redundant disk array controller) need a little more.

If you just go through and enable the DMP driver you’ll be able to see, format, and use the storage from the DS4800… but what you’ll eventually notice is that the LUN will continually failover to another path using a different storage controller.  This is called thrashing and its not a good thing.

To stop the thrashing you’ll need to do one of two things:  1) tell the DS4800 to disallow AVT (automatic volume transfer) or 2) tell the XenServer to pick a path and stick with it until a failover is necessary (i.e. STOP THE ROUND ROBIN)

The first option of disabling AVT is actually the easier of the two and is done on the storage controller itself.  Don’t worry you can set this by LUN or even by host so you won’t be in danger of tampering with your other SAN attached systems.  The way to do this in the IBM DS4800 is to change the host type to something that doesn’t perform AVT.  The host type of Linux VMware Cluster or LNXCLUVMWARE should do the trick.

The second method of chaning the driver over from DMP to MPP is not as easy.  If you go out and do some searches on the matter you’ll find that Citrix states in CTX121364 ”This is the real RDAC driver supported by IBM and other vendors” about the MPP driver.  What you won’t find in that article are steps to enable and actually use the MPP driver.  Your searching might even land you at CTX118791 where you might find specific reference to the IBM DS4xxx.  But once again you’ll be hard pressed to actually find out how to get this thing working.

I recently spoke with the XenServer support team and got some instructios on how to enable the MPP driver in place of the DMP driver.  Its not an official CTX article yet but my understanding is that they are working on it.  Even when it does get released it won’t be for the faint of heart.  I should also point out that the cute little “4 of 4 paths active’ notification that you see in XenCenter ONLY works with DMP so once you switch over to MPP you’ll lose that information.

Changing the host type on the storage controller is definately the easiest and it will allow you to continue to see the multipathing information in XenCenter.  But it definately seems to struggle with performance.  I think this is largely due to the fact that XenServer is still trying to use round robin multipathing but the IBM DS4800 is not allowing access to the LUN through one of the controllers… meaning up to 50% of your I/O requests from XenServer will not be fullfilled.

Some of my biggest complaints about XenServer when compared to VMware ESX was that XenServer was lacking some of the features that have become critical to the successful adoption and implementation of a hypervisor.  Specifically, XenServer has been missing dynamic load balancing  and snapshot capabilities.  Also included in XenServer 5.5 is support for Active Directory authentication through XenCenter.  You can read the full list of new features in the release notes found here:  http://support.citrix.com/product/xens/v5.5/

Now it would be easy to argue that all of these features have been available from VMware since the inception of VI3 through ESX 3.0 and VirtualCenter 2.0 which was originally released in June of 2006!

So is this just Citrix playing catch up?  Are they really three years behind VMware?

Citrix XenServer 5.0 does support PXE booting, in fact things like Citrix Provisioning Server depend on PXE booting.  The issue is that technologies like Citrix Provisioning Server make use of DHCP options 066 and 067.  These options respectively tell the client the address of the TFTP server and which image file should be used to boot from.

While I can’t speak for Altiris or Windows Deployment Services I can say that even as of Citrix XenServer 5.0 support automated operating system deployment technologies is limited.  Several of these automated operating system deployment technologies make use of DHCP options other than the traditional 066 and 067 used by PXE booting.  In fact one some of the more common DHCP options that appear to be unsupported by Citrix XenServer 5.0 are DHCP options 043 and 060.  More specifically they do not appear to be supported by the Etherboot project which is the component behind the networking booting found within Citrix XenServer 5.0.

I was unable to obtain a comprehensive listing of what DHCP options are in fact supported by Citrix XenServer 5.0 and/or the Etherboot project so I would be very interested to find out if anyone has any insights.

There are many posts lingering around the Citrix support forums on this exact thing:

• http://forums.citrix.com/thread.jspa?threadID=150858
• http://forums.citrix.com/thread.jspa?threadID=154029
• http://forums.citrix.com/thread.jspa?threadID=153909