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Date: Wed, 03 Oct 2012 20:17:47 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] O_DIRECT on glusterfs (was Re: [PATCH v9 4/4]
 block: Support GlusterFS as a QEMU block backend)
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To: Anand Avati <aavati@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Vijay Bellur <vbellur@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, Harsh Bora <harsh@linux.vnet.ibm.com>, Amar Tumballi <amarts@redhat.com>, qemu-devel@nongnu.org, "Richard W.M. Jones" <rjones@redhat.com>, Blue Swirl <blauwirbel@gmail.com>, Avi Kivity <avi@redhat.com>, bharata@linux.vnet.ibm.com, Daniel Veillard <veillard@redhat.com>

Il 03/10/2012 19:58, Anand Avati ha scritto:
>>
>> I think these 3 lines should be removed.  We're bypassing the host
>> buffer cache just by virtue of using a userspace driver, and that's what
>> cache=none cares about.
> 
> O_DIRECT also has an effect on the behavior of the "client side" (the
> part within the qemu) of Gluster stack as well. I presume the intention
> of O_DIRECT is to minimize use of memory (whether as host' page cache or
> buffered data in user space). To that end it is a good idea to leave
> O_DIRECT flag set.
> 
> The behavior of whether gluster bricks need to get the O_DIRECT
> propagated or not is a different issue. We are exploring the possibility
> of not sending O_DIRECT flag over the wire to mimic NFS behavior. That
> would be independent of the qemu block driver setting the open flag.

What is the effect of O_DIRECT on the client exactly?

Paolo