Re: [Xen-users] dom0 performances

[Tom Brown <xensource.com@xxxxxxxxxxxxxxxxxxx>]

On Mon, 16 Jul 2007, shacky wrote:

> >  It depends how much IO the domUs will generate. 2xSATA isn't that fast...
>
> Couldn't we estimate it? ~20, ~50, ~70?
70 random seeks per second should be reasonable (7200 rpm = 4ms average 
rotational latency, 8ms average seek time -> 12ms per random seek -> 
would suggest that 83 completely random I/O's is achievable on a 
standard 7200rpm drive with 8ms seek times. However, that's basically 
just another meaningless number....
It's hard to have any real idea of how many I/O's are random and how many 
are sequential. It's also dramatically affected by how much memory each 
domU has for disk caching and of course, but what the domU workload is.
Looking up keys in a 20 Meg mysql database is going to be completely 
cacheable. Full table scans (or even selecting a few thousand random rows) 
of a 10 or 100 gigabyte database will drive almost any I/O subsystem nuts 
(and if you have a massively parallel drive system, just keep adding 
clients, it is still overloadable). Logging data to a mysql table will be 
pretty much sequential writes and schedulable at the kernels convenience. 
Running a webserver to serve a couple meg of static files is completely 
cacheable.
Running a LAMP setup on a domU that has only enough memory to actually 
load apache/mysql into memory is going to be hard on disk (because there 
won't be any disk caching).
And this IMHO is the biggest problem with virtual machine hosting. 
Accounts tend to be sized by memory, and the clients thus slot themselves 
into the smallest amount of memory they can survive in, and consequently 
burn out the disk subsystem... and AFAIK, xen (and probably most other 
virtual hosting solutions), do not have good control/scheduling/limiting 
of/over the amount of disk activity caused by a domU. Allocating a drive 
to each domU is wastefull, limiting, and counter to the goals of 
virtualization... but it does greatly reduce the effect of the domUs on 
each other.
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