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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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?

Basically, yes.

The domU writes data to a virtual device which is provided, in some way, by 
dom0.  If you're using HVM then your guest might be using an emulated version 
of a PATA drive (for instance).  If you're using PV then the guest is using 
an optimised, Xen-aware block driver to access the virtual device.  PV block 
devices have better performance and can also be used in some HVM guests; for 
non-Linux HVM you're basically stuck with the slower emulator.

You can basically export anything that's a block device in dom0 as a virtual 
disk, including a RAIDed volume, LVM volumes, etc.

As Tom said, doming RAID in dom0 and hiding it from the guest will halve the 
number of disk requests that the domUs have to issue.  It'll also simplify 
the configuration of the domUs (if you change the RAID setup in dom0 they 
don't have to know about it).

> 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 ....

If you're using RAID to keep the host up whilst a disk drive explodes, then 
RAIDing the guest swap too may make sense.  Guests do all their own swapping, 
so if they have swap configured then it is a necessary part of their running 
state.

> 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 ...)

Pretty much any filesystem is good...  There have been some problems with XFS 
and Xen so possibly I wouldn't recommend that.  ext3 seems to be widely held 
as the most solid Linux filesystem these days, if not the most technically 
advanced, so I guess if you don't have any specific reason to choose another 
one then that would be my recommendation.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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