Re: [Qemu-devel] O_DIRECT on glusterfs (was Re: [PATCH v9 4/4] block: Support GlusterFS as a QEMU block backend)

[Paolo Bonzini]

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