Re: [User question] Huge buffer size on KVM host

[Avi Kivity <avi@redhat.com>]

On 08/15/2012 11:06 AM, Martin Wawro wrote:
> Hi all,
> 
> We have noticed a strange thing in one of our setups which uses KVM for
> virtualization.
> The size of the buffer cache turns out to be rather large, here is what
> 'top' on the host
> has to say about it:
> 
> Mem:  12274084k total, 12202860k used,    71224k free,  2991728k buffers
> Swap:  8191992k total,    20232k used,  8171760k free,    44392k cached
> 
> As the host machine itself does not run any data intensive services, we
> assume that
> this is due to the guest VM which receives/sends about 200 GB of data
> per day of which
> about 35G have to be stored on disk (disk array connected to the host
> and used inside
> the guest via virtio). We did not run into problems with this setup, it
> basically runs stable
> for more than a year now (the buffer size never went past 30% of the
> total memory).
> However, we are not sure if the huge size of the buffer cache is
> something one would
> expect here. If this is the case, I would really love to know the reason
> behind it and if
> it has indeed something to do with KVM/virtio. Any enlightenment here
> would make
> my day.
> 
> Our setup:
> --------------
> 
> Host:
>   IBM BladeCenter HS22, 12 GB RAM, Xeon QC CPU
>   CentOS 5.5 (64-bit) running 2.6.18-194.11.1.el5
>   kvm-83-164.el5_5.25 (qemu 0.9.1, modules 2.6.18.194.17.1)
>   libvirt 0.6.3
> 

How is you storage set up?  Files (which format?) or logical volumes?
what's the cache= setting?


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