[Qemu-devel] bad performance accessing nfs exports on qemu host

[Qemu-devel] bad performance accessing nfs exports on qemu host

[Henning Sprang]

Hi,
First of all, this is more a question about qemu usage, not development,
but the link to the qemu user forum on the qemu website
( http://www.dad-answers.com/qemu-forum/index.php )is broken (404), and
I don't see any other place to get qemu help for users. If there is a
better place for this question, please tell me so.

I am trying to use qemu for helping me with the development of an
automatic installation software for Debian Linux (FAI, that is).

Therefore I need a qemu guest system (FAI install client) mounting it's
root file system from an nfs server on the qemu host, and unpacking a
tar.gz file from that root into a directory from it's "local" virtual
harddisk.

While all the other services involved (dhcp, dns, ssh) seem to work with
no problems, I have serious performance trouble with NFS when I want to
copy  or unzip the file mentioned. I get a mount, and, when copying the
file, the whole qemu guest system turns incredibly slow, and it takes
approximately half an hour to copy a 40M file from the nfsroot to it's
local disk. In that time, also any command given on the qemu guest's
shell executes extremly slow. Copying files with ftp is much faster.
There is not big  CPU load or anything to be seen on neither guest or
host. So finally everything works, and my automatic install comes to an
end, but with a very long delay, taking a sum of more than an hour, of
which most of the seems to be spent on just copying a 40M file.

There's no firewall blocking NFS access on the host, I restarted all
services after starting the qemu vm, so all services are restarted with
a tun0 device existing (not doing so seems to confuse some of the
services mentiones). I cannot see any messages in any log files which
would help me getting rid of that problem.

What could it be causing this behaviour? Is there a known problem with
using especially NFS between the qemu host and client?

Please tell me if you need any further information to answer my
question, I might have forgotten an important detail.


Thanks in Advance,
Henning

Re: [Qemu-devel] bad performance accessing nfs exports on qemu host

[Henning Sprang]

Hi,
I found some answers/help to my questions by further looking:

On Tue, 2005-10-04 at 15:41 +0200, Henning Sprang wrote:
> Hi,
> First of all, this is more a question about qemu usage, not development,
> but the link to the qemu user forum on the qemu website
> ( http://www.dad-answers.com/qemu-forum/index.php )is broken (404), and
> I don't see any other place to get qemu help for users. If there is a
> better place for this question, please tell me so.

The Link on the website is really broken, but I found the user forum on
http://qemu.dad-answers.com/ - it could help users and developers a lot
if that link could be fixed.

I also wonder why there is no Mailing list for users.


> 
> I am trying to use qemu for helping me with the development of an
> automatic installation software for Debian Linux (FAI, that is).
> 
> Therefore I need a qemu guest system (FAI install client) mounting it's
> root file system from an nfs server on the qemu host, and unpacking a
> tar.gz file from that root into a directory from it's "local" virtual
> harddisk.
> 
> While all the other services involved (dhcp, dns, ssh) seem to work with
> no problems, I have serious performance trouble with NFS when I want to
> copy  or unzip the file mentioned. I get a mount, and, when copying the
> file, the whole qemu guest system turns incredibly slow, and it takes
> approximately half an hour to copy a 40M file from the nfsroot to it's
> local disk. In that time, also any command given on the qemu guest's
> shell executes extremly slow. Copying files with ftp is much faster.
> There is not big  CPU load or anything to be seen on neither guest or
> host. So finally everything works, and my automatic install comes to an
> end, but with a very long delay, taking a sum of more than an hour, of
> which most of the seems to be spent on just copying a 40M file.
> 

I found a workaround for that - when I enable NFS over TCP by giving my
install client a line like this with dhcp:

  option root-path
"/usr/lib/fai/nfsroot,rsize=8192,wsize=8192,acregmin=1800,acregmax=1800,acdirmin=1800,acdirmax=1800,tcp";

- most important is the last part "tcp" - then everything goes much much
faster. It seems like qemu doesn't work well for udp packets, maybe the
developers have a better idea and explanation for that?!

Anyway, have a nice day - mine is made by this great thing I found, now
I can use qemu for real nice testing of FAI installation without
spending lots of money for hardware.

Henning