Re: [Xen-users] Cheap IOMMU hardware and ECC support importance

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 07/02/2014 12:29 AM, Adam Goryachev wrote:
> On 02/07/14 05:37, lee wrote:
> > Adam Goryachev <mailinglists@xxxxxxxxxxxxxxxxxxxxxx> writes:
> >
> > > PS, this is my personal advantage of using virtualisation... If I need
> > > n servers to run all my VM's, then with n+1 I can survive a failure of
> > > a dom0 (albeit with a reboot of the VM's being run there at the
> > > time). I still get to save on overall hardware requirements, as well
> > > as being able to live migrate for scheduled hardware maintenance/etc.
> > For how many servers do you need one standby?
> >
> > Wouldn't you be better off not using a standby but moving each of the
> > VMs from a failed server to another one that still works?  One more or
> > less VM on a number of servers probably won't hurt?  You could even plan
> > for it and spare resources on existing servers for this, or enjoy things
> > running faster than required until you need the additional resources for
> > a migrated VM.
> >
> > You'd need one server less per so many servers.  The less servers, the
> > less can fail.
> Correct, mostly.
>
> If I decide I need 4 dom0's to allow my VM's to work properly, then I
> will use 5 dom0's, but the 5th is not idle, I will assign some VM's
> there all the time. On failure, I re-balance the VM's from the failed
> dom0 to the remaining dom0's.
>
> Essentially, as long as total RAM requirements of all VM's is less than
> the amount of RAM available on the remaining dom0's, then "service" can
> continue. This is one limitation of xen, (can't overcommit RAM), which
> limits the number of survivable dom0 failures.
>
> Of course, performance may become degraded due to the over-provisioning
> of CPU (or RAM if it was possible).
>
> Using this guideline, the minimum number of dom0 should be 2, but for
> me, I prefer a minimum of 3, assuming at least 3 VM's.
>
> As such, I tend to buy "desktop" class machines for dom0, with single
> CPU, and not so much RAM, but just buy more of them. Instead of one
> server for $3000, I'll get 3 servers for $1000 each. I'm convinced the
> chance of all three desktop machines failing at the same time is much
> lower than the single server class machine failing (assuming dual power
> supply, dual CPU, etc...).
Of course not all failures are the same. Data corruption due to lack of 
ECC RAM or just everyday disk sector failures is an issue.
Gordan


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