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September 2011

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Subject:
Re: Need KVM HD settings advice
From:
Stephan Wiesand <[log in to unmask]>
Reply To:
Stephan Wiesand <[log in to unmask]>
Date:
Tue, 20 Sep 2011 16:56:07 +0200
Content-Type:
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On Sep 20, 2011, at 14:48 , Nico Kadel-Garcia wrote:

> On Tue, Sep 20, 2011 at 3:05 AM, Vladimir Mosgalin
> <[log in to unmask]> wrote:
>> Hi Todd And Margo Chester!
>> 
>>  On 2011.09.19 at 18:03:08 -0700, Todd And Margo Chester wrote next:
>> 
>>> What I need help with is getting the optimum performance
>>> settings while converting over my old hard (virtual) drive.
>>> 
>>> This is what I have gathered from these parts as to the best
>>> settings:
>>> 
>>> - controller: virtio
>>> - kvm option: cache=none
>>> - qcow2 disk format with metadata preallocation
>>> - create your disk image with:
>>>      qemu-img create -f qcow2 -o \
>>>      size=40000000000,preallocation=metadata vdisk.img
>> 
>> If you don't need snapshots and such, you might get better performance
>> with LVM volumes for storing images, connected as "raw images". At least
>> that's only thing that I'm using in production and it works well.
> 
> LVM has its uses. But the ability to re-allocate space without having
> to manipulate your partition tables is *vital* in a dynamic
> environmemnt, and it's a lot easier to do with image files.

I disagree. There's no need to manipulate partition tables in order to provide LVs as backing store to VMs. Extending a LV is as easy as enlarging a backing file. LV data alignment can be specified, at least with SL6.

I see no disadvantage in using LVs for VM backing store, other than having to learn about LVM. And it does avoid the significant overhead of going through a second filesystem layer. At least potentially, it also avoids double caching.

Since we introduced Linux-(Xen-)hosted VMs with SL5, we too have backed all our VMs - hundreds - with logical volumes. And we haven't changed that with SL6 and KVM.

> Tuning nthe partition the images reside on, now *that* is invaluable.
> Turn off noatime, use a fast and simple file system.

Well, just don't use a filesystem... NB noatime tunes the filesystem, not the block device (partition or LV) it resides on.

[...]


-- 
Stephan Wiesand
DESY -DV-
Platanenenallee 6
15738 Zeuthen, Germany

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