Mike O''Brien wrote:> The main Xen mailing list ("xen-users") generally advises
> users to limit the memory on dom0 to 2Gb or less. Apparently the
> general version of Xen has troubles with this.
I wouldn''t say that, this is a general recommendation
we give when someone is setting up a dom0 with zfs
and going to use it to run domUs.
> What''s the corresponding advice for the Xen in OpenSolaris,
> Nevada b87 in particular? I''ve got an X4600 with 32Gb of physical
> memory on it. I was originally planning to have dom0 be the general-
> purpose compute instance, and also have it serve a collection of
> Linux guests.
>
> Am I better off having a smallish 2Gb OpenSolaris dom0,
> a giant OpenSolaris domU for most compute jobs, and a bunch of
> smaller Linux domUs for the Linux users?
It all depends on what you are trying to do...
In general, dom0 should not be treated as a compute node.
It should be use for providing IO services to guests,
and for managing guests. You can though.. It will
still work, but not as well in some cases.
Personally, I would start with a smallish dom0, 2G
if you using zfs, you can probably go down to 512M if
your not using zfs or HVM domains, and even less if you
minimize your dom0. I would also pin the dom0 CPUs and limit
the number of dom0 cpus to 2 (4 if a 32-way or more).
Shut off any services you don''t need that take up a fair amount
of memory, e.g. cde-login, webconsole.
Setup your guests so they don''t run on the dom0 cpus.
i.e. if you limit dom0 to 2 cpus and have 16 cpus
total, put
something like the following in the guest configs
cpus = "2-15"
For the big Solaris domU, you might want start it with a
lot of memory and CPUs then after it''s booted, remove some
memory and CPUs. This way you can grow it when needed,
then shrink it back down when not being used. You can''t
grow it larger than what you booted it with.
The linux domUs should be PV kernels if possible.
> And if so, for heaven''s sake why? What purpose does this
> limitation serve?
You can give dom0 all the memory. But, when you balloon down,
the dom0 VM is still has to have the tracking info for it''s
original memory. i.e. the more memory, the slower the VM code
path is.
MRJ