Re: [Xen-users] RAID-1 strategy for a Xen/CentOS server?

[Tom Brown <xensource.com@xxxxxxxxxxxxxxxxxxx>]

On Fri, 7 Dec 2007, Bob Tomkins wrote:

> Hi Tom,
>
>
> because vbd are slower than "real-world" devices, I believe you want to
> > move the mirroring into dom0, so that the domU's aren't sending the same
> > data through the hypervisor twice... beyond that, all the rules are the
> > same as they would be in a non-XEN config. (This just makes plain sense,
> > there's less work to be done.)
> >
> > There may be situations where it makes more sense to do the raid in the
> > domUs, but you listed "optimize performance".
>
> Thanks for the reply.
>
> Yes, performance is my primary concern, at least during my planning.  (One
> DomU
> will consist of Zimbra/CentOS, which I understand to be an I/O hog, so I'm
I was going to say "add memory", and that's always a help with respect to 
reducing disk I/O ... but zimbra seems to be a mail-server platform ... 
and MTA software often _forces_ disk I/Os via fsync() type calls. Still 
making sure that the VM has lots of memory will reduce the disk reads, and 
allow non sync()'d I/O to be cached for a bit longer and written to disk 
when convenient.
frankly, IMHO and in my experience, things that eat up disk I/O aren't 
particularily well suited to running on virtual machines. XEN generally(*) 
is for cutting up a big machine into smaller pieces which can be more 
conveniently administered... partly due to the isolation between 
machines...
... but disk drives do not "isolate" well unless you are dedicating 
spindles to individual domains... if a domain starts maxing out it's 
attached drives, any other domain using that drive is going to see it's 
load average go up as processes trying to use those drives get stuck in a 
long queue waiting for disk I/O.
(* - there are situations where inserting a "shim" between the physical 
hardware and the O/S are usefull... being able to backup a windows box, or 
replicate it via DRBD in real time are examples, that require having a 
"virtual" layer. The consistency of the virtual machine is also good, as 
you can pick up a domU and start it on another physical box and see the 
same virtual machine.)
> Am I correct in my understanding that each DomU contains an instance of
> OS/kernel + app binaries in it's own virtual volume/file space, but that
> *data* (effectively, *any* dynamic content) is written by the DomU processes
> to/from the Dom0 hypervisor's volume/file space?
>
> How about swap?  Dom0 of course has its swap -- and could/should be RAIDed,
> but what about  GuestOS' swap?  I honestly haven't gotten that far yet ....
What is the point in "raid'ing" swap space? There's nothing in swap you 
really need to preserve. If your target is trying to increase 
reliability... I'd find other approaches ... but even so, the same logic 
applies, either have the VM swap to a dedicated vbd or have it do LVM on 
it's existing vbd's and swap to one of those ... the same raid rules 
apply.
This might be relevent to your HW vs SW raid. Generally HW raid will make 
it simpler to replace a drive without having to reboot the system... but 
it tends to have constraints of it's own...
> And, if DomU hosts the RAID mirror, what's the recommended file system
> choice -- or is that dicated by Xen as a preference (I haven't got to that
> either yet ...)
AFAIK, none of this has anything to do with xen. In theory the folks 
behind your target software (like zimbra) should have recommendations.
-Tom


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