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Re: [Xen-users] dom0 performances

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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