Re: Slow I/O on NetBSD domU (on Debian dom0)

[Stefano Marinelli <postnet%dragas.dyndns.org@localhost>]

Hi Greg,

>  intensive I/O task (like "tar xjvf pkgsrc.tar.bz2"), the NetBSD domU
>  starts to lag. ssh connections take ages to be accepted, and generally
> 
> How are your "xbd" instances backed on the dom0?

The dom0 is a Debian Squeeze amd64 system, two SATA 1.5TB disks, a big raid 
with LVM over it, and the NetBSD xbd are LVM volumes.

> I use NetBSD as the dom0, with files as virtual disks, via vnconfig
> using the normal scripts.  I find about a <10% penalty on dd from the
> file compared to dd from the raw dom0 disk, and then another <10%
> penalty from the file on dom0 to the rxbd on domU.  So things seem good.

This is the same thing I've noticed when using NetBSD as dom0. It seems 
generally more efficient than Linux, but here I had no choice :(

> Things to check:
> 
>  can you dd from the backing store for the xbd at a reasonable speed?
> 
>  can you dd from the dom0 rxbd at a reasonable speed?
> 
>  what does 'systat vmstat' on the domU show during the heavy IO?
> 
>  what does the Linux equivalent show on the dom0?

I will try all this as soon as the machine will be under lower load and will 
report results back

> I read a paper or web page long ago that claimed NetBSD as dom0 was
> particularly efficient, but I can't find a reference to it quickly.

I've never that paper, but my real-life experience confirms that. NetBSD as 
dom0 is MUCH faster than Linux.

Thanks,
Stefano