[User question] Huge buffer size on KVM host

[Martin Wawro <wawro@digitalmedics.de>]

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

Guest:
  Ubuntu 10.04 (64-bit) running 2.6.32-27
  virtio for storage and network


Best regards,

-- 
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Martin Wawro           |                         Digital Medics GmbH 
Managing Director      |  Otto-Hahn-Str. 15, 44227 Dortmund, Germany
Tel. +49-231-9742-6622 |                      Fax: +49-231-9742-6623
Key: 0xB0A225BD        |        Registered at AG Dortmund, HRB 19360
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