Re: [Xen-API] XCP + RAID

[Dave Whitla <dave.whitla@xxxxxxxxxxxx>]

Hi Scott,

I understand exactly where you're coming from. At work I use multimillions 
worth of bleeding edge blade hardware in our $50,000,000 datacenter deployment 
but at home I play around with Xen on my ASUS X58-based system.
I think some people cannot differentiate need from desire. I am running more or 
less your setup in one of my Xen deployments. I'm not sure what George has 
against Debian but thats what I've been relying on in production since 1997 and 
I've run just about every operating system known to man at some time or another.

Anyway...

You may or may not realise that there is a distinction between XCP and the 
XCP-derived XAPI tool stack.
It is the XAPI tool stack that you would install on a Debian machine. XCP on 
the other hand is a prefabricated open-source equivalent of XenServer.
XCP is actually the whole box and dice, so when you install XCP from the ISO, 
it installs a complete operating system (based on CentOS) along with some 
Citrix-patched components (such as their own modified LVM) and the canonical 
XAPI tool stack.
Once you edit the management interface IP configuration you have a working 
XenServer clone (albeit without the pay-to-play features).
This is the simplest idiot-proof way to play with a Xen-hypervisor system.

If you are a masochist like myself, you may prefer to more deeply understand 
how things work or you may just have a personal idea of how a cluster of 
machines should operate which differs significantly from those of the guys at 
Citrix.
Either way, through the work of some Citrix employees, the XAPI tool stack is 
in the process of being ported to Linux distributions other than XCP, Debian 
being the first targeted.
I say "in the process" because it is far from ready for production (or even 
home use) unless you want to really get your hands dirty. It's an active 
project (known as Kronos) but I'd say about 50% of it plain doesn't work yet.

Now to your actual issue.

You already know that the "RAID" on your mainboard is bogus. No need to harp on 
that except maybe to the marketing people who continue to mislead everyone.
You've researched Linux software RAID but now you're stuck. If you really MUST 
have RAID then XCP may be forced to do it but upgrades may be problematic.
XCP is after all just CentOS anyway and you can boot CentOS from a software 
RAID volume.

If you MUST use Debian you need to know about some issues booting from a 
software RAID volume on more recent Debian versions. There is a fairly well 
known (by sufferers) race condition during boot involving LVM and the MD 
subsystem where subsequent steps in the boot sequence do not wait long enough 
for the device mapper to build the RAID set before attempting to mount the root 
filesystem. This results in the boot failing with messages about a non-existent 
root filesystem partition. The workaround is involved and you need persist with 
applying it only if you really want to use Debian instead of XCP.
CentOS may exhibit the same behaviour - I don't use it myself so I can't advise 
you there. If it does, then the workaround for it will be as for Debian.

XCP, being an attempt at a noob-proof system, has no support for "fringe" Linux 
features like software RAID. "No support" doesn't mean can't be done. Citrix 
just doesn't want the headache of supporting too-varied deployments and by 
requiring plain SCSI disk presentation to the OS (a-la true hardware RAID) they 
can make any RAID issues someone else's problem.

Given all the foregoing I assume that what you actually want to do is get a 
free XenServer-like single node up and running with enough user-friendliness 
that someone else at the office can deal with issues when you're not around, 
and enough fault-tolerance that a weekend power-outage won't cost you the next 
week in rebuilding everything.
I think the software RAID solution in this scenario is perfectly acceptable. I 
also think that Kronos is way too green to pick right now. Because of this, and 
because XAPI is required by any of the half decent GUI management tools 
(VirtManager excepted), I recommend you build an XCP installation rather than 
an XAPI-on-Debian installation.
Your users won't know its an inferior Linux distro ;).

You can convert your disk storage to software RAID1 after you have installed 
XCP and got it all sorted out on a single disk.
When you do this you will need to consult the relevant documentation for the 
procedure for CentOS.
The higher level XCP elements won't know or care that your partitions are on 
/dev/md0 instead of /dev/sda and it is possible to hide even this.
The conversion will not be without a fair bit of messing around and you may 
need to work around the aforementioned boot race condition.
If this is too much effort to go to then it may just be cheaper (than labour) 
to buy an Areca, HighPoint or other IOP-based SATA/SAS RAID card. Mine cost 
about AU$600.

Dave
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