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Re: [Xen-users] Default disk I/O scheduler in linux guest

To: Jia Rao <rickenrao@xxxxxxxxx>
Subject: Re: [Xen-users] Default disk I/O scheduler in linux guest
From: Javier Guerra <javier@xxxxxxxxxxx>
Date: Thu, 5 Feb 2009 09:52:54 -0500
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On Wed, Feb 4, 2009 at 5:24 PM, Jia Rao <rickenrao@xxxxxxxxx> wrote:
> Hi, all,
> I just found out that xen assigns the NOOP disk scheduler for linux guest
> OSes. Dom0 uses cfq scheduler (it is the linux default).
> Is there a reason for xen to turn off disk request merging in the quest OS
> by selecting a NOOP scheduler?
> Is it because the request optimization will be performed in dom0 or VMM ?

it's an appropriate default.

everything that virtualizes the IO benefits from using NOOP scheduler.
 the point is that any (re)ordering done by the guest would be useless
when the underlying layers (Dom0 in this case, a SAN block device in
others) mangle the IO requests from several guests.  not only you save
CPU cycles by not trying to be clever on the DomU, but also pushing
the requests as early as possible to the lower layer, the best
optimisations can be done at that layer.


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