[PVE-User] Very slow server (RAW IDE discs and KVM)

Erik van Ast Erik.van.Ast at suzohapp.nl
Mon Aug 8 12:04:44 CEST 2011

Hi all,

Sorry for the previous 2 messages I've send this morning. I forgot the
change the subject and the rest of the message....

@Fabian and @CoolCold thank you for your reply.
I have connected with putty (SSH) and ran #top.
I see 4 CPU lines (CPU0-CPU4) and the max I have seen now during 5
minutes of watching it is 15%, but most of the times, it's 1 Cpu doing
4% and 1 doing 2%, so I presume that's not the problem?

I also edited the .conf file located in /etc/qemu-server and I have
tried both " ide0: local:108/vm-108-disk-1.raw,cache=writethrough" and "
ide0: local:108/vm-108-disk-1.raw,cache=none" but with both configs I
didn't see any major improvements. The virtual server has been installed
with 2 virtual discs, 1 of 40GB and the second with 200GB.
The machine is especially slow when copying large data over, than the
speed drops to about 30-70 Kbyte/sec instead of at least 20 Mbyte/sec...

Anyone seen this before? Help is greatly appreciated...

Kind regard,

Message: 2
Date: Fri, 05 Aug 2011 13:28:47 +0200
From: info at fabianschuetze.de
To: <pve-user at pve.proxmox.com>
Subject: Re: [PVE-User] Very slow server (RAW IDE discs and KVM)
Message-ID: <8f9fd64c580b8de77bb9f6babe6d8903 at fabianschuetze.de>
Content-Type: text/plain; charset="utf-8"


Login via ssh / Console to your proxmox host and execute 


You should see a "x.x%wa" in the "Cpu" line.
If it is very high, the
HDD's are too slow/busy to write the files. 


On Fri, 5 Aug
2011 13:10:54 +0200, Erik van Ast wrote: 

> Hi all,
> We have a
PROXMOX 1.8 server running on a "4 x Intel(R) Core(TM)2 Quad CPU Q9650 @
3.00GHz" CPU with 8GB RAM installed. At the moment 3 virtual servers are
running on it (OpenSuse 11.2, Windows 2003 and Windows 2008) and one of
my servers (Windows 2003, 2GB of RAM, 2 RAW IDE discs) is being
extremely slow when writing data to it. We want to use this machine as
file server, so we want to copy 180GB of data to it. The copying of data
is almost finished now, but it took about 4 days to complete and we are
on a 1Gbit connection internally so that shouldn't make a difference. I
saw that the machine is especially slow when copying large data over
(like PST files), the speed than drops to 10-20Kbps instead of the
multiple Mbps it should do. 
> At the moment 6.37 of the 8GB have
been takes by the machines so RAM shouldn't be a problem and total CPU
is 50% now, so that should also be no problem. 
> When running task
manager on the virtual server itself, nothing special is seen. No high
CPU, no high RAM but the server is almost unresponsive. As soon as the
server is finished copying, everything is back to normal speed 
> Has
anyone ever seen this and does anyone know how to solve it? 
> Kind
> Erik

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Message: 3
Date: Fri, 5 Aug 2011 15:36:17 +0400
From: CoolCold <coolthecold at gmail.com>
To: Erik van Ast <Erik.van.Ast at suzohapp.nl>
Cc: pve-user at pve.proxmox.com
Subject: Re: [PVE-User] Very slow server (RAW IDE discs and KVM)
<CAGqmV7rJ8qTS9tLXkUQ9ann+Ot3ofDyYiekS5ePKfxrkz3OR6w at mail.gmail.com>
Content-Type: text/plain; charset=windows-1252

I'm not expert in Proxmox (never used or even installed) but regarding
KVM - may be you should check "cache" policy for that machine(s) ? One
day I was installing Win7 inside KVM container and changing cache mode
helped me (i may be wrong but i set it to "none").

Quoting Tuning KVM[1]

QEMU also supports a wide variety of caching modes. Writeback is useful
for testing but does not offer storage guarantees. Writethrough (the
default) is safer, and relies on the host cache. If you're using raw
volumes or partitions, it is best to avoid the cache completely, which
reduces data copies and bus traffic:

 qemu -drive

Hope this will help you.

[1] http://www.linux-kvm.org/page/Tuning_KVM

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