[DRBD-user] Slow Reads on VM - Xenserver and DRBD

Felix Frank ff at mpexnet.de
Tue Jul 24 09:40:17 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On 07/24/2012 06:14 AM, Christian Balzer wrote:
>> > Can you help me? Windows VMs have shown the same issue. 
> That clearly looks like some Xen deficiency or configuration issue, can't
> help you with that, sorry.
> 

That reminds me - I've once heard a talk on just why Xen has I/O
performance issues. It's because of the way syscalls are handled among
DomU kernel, Dom0 kernel and hypervisor.

KVM has advantages there. There's even an experimental hypervisor(?)
called Xenner that allows running Xen domains on a KVM host, with better
syscall handling than Xen itself.

Note that such deficiencies (and/or advantages) usually apply to certain
workloads only. I advise to do more than simple throughput tests. If you
*do* measure throughput, I believe that you should transfer more data
than fits a 3 second window.

Finally: hdparm? Isn't that limited to the old IDE kernel drivers? Are
you using those? If so - why?

Cheers,
Felix



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