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Re: [Xen-users] how can empty xen disk cache??

  • To: Pasi Kärkkäinen <pasik@xxxxxx>
  • From: Radhya sahal <rad_cs_2006@xxxxxxxxx>
  • Date: Sat, 23 Jul 2011 18:46:58 -0700 (PDT)
  • Cc: xen list <xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Sat, 23 Jul 2011 18:48:33 -0700
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thank's Mark and Pasi,
i use centos and xen ,either dom0 and vm run centos 5.5.
in the vm i run java application which it reads e.g( text file ,table in database,....).
in the first read get actual total run-time but if i try to read more than one . the file be in cache and get a very very short time ..!!!
although i try to empty the vm cache and physical(dom0) cache!!!
echo 3 >/proc/sys/vm/drop_caches
i want to read more than one to calculate the the right average run-time for the vm resource allocation and  compare this averages with other vm resource allocations ???

--- On Sat, 7/23/11, Pasi Kärkkäinen <pasik@xxxxxx> wrote:

From: Pasi Kärkkäinen <pasik@xxxxxx>
Subject: Re: [Xen-users] how can empty xen disk cache??
To: "Mark Pryor" <tlviewer@xxxxxxxxx>
Cc: "Radhya sahal" <rad_cs_2006@xxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>
Date: Saturday, July 23, 2011, 6:44 AM

On Fri, Jul 22, 2011 at 07:11:57PM -0700, Mark Pryor wrote:
>    --------------------------------------------------------------------------
>    From: Radhya sahal <rad_cs_2006@xxxxxxxxx>
>    To: xen-users@xxxxxxxxxxxxxxxxxxx
>    Sent: Friday, July 22, 2011 5:36 PM
>    Subject: [Xen-users] how can empty xen disk cache??
>    hi
>    I am new user for xen
>    i have VM and i run java application on it ..
>    i want to empty cache for vm
>    i do that with
>    echo 3 >/proc/sys/vm/drop_caches
>    and the same command in physical machine
>    but there is not any effect
>    i think the vm have own cache in xen
>    how can empty xen disk cache??
>    thanks
>    check options in /etc/sysconfig/xendomains or (etc/default -- in debian)
>    and turn off save/restore.

I don't think the question has nothing to do with save/restore.

Dom0 does not usually cache domU disk IO, unless you're using file: backend.
Which disk backends are you using?

tap:aio and phy: backends bypass the dom0 page cache,
and thus those backends don't have any caching in xen dom0.

Xen hypervisor itself does not have cache of any kind.

-- Pasi

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