[vbox-dev] [PATCH] open virtual disk file with O_DIRECT
Frank Mehnert
frank.mehnert at oracle.com
Wed Mar 2 07:48:29 GMT 2011
On Wednesday 02 March 2011 05:53:52 howard0su wrote:
> On Wed, Mar 2, 2011 at 5:17 AM, Frank Mehnert <frank.mehnert at oracle.com>
wrote:
> > On Tuesday 22 February 2011 11:24:30 Howard Su wrote:
> >> I am not sure how big diff this change will bring in. But I think this
> >> is worth trying. I enable this unconditional because i believe the
> >> most write for disk file doesn't need cache in host (guest already
> >> handle it.)
> >>
> >> Index: VD.cpp
> >> ===================================================================
> >> --- VD.cpp (revision 35998)
> >> +++ VD.cpp (working copy)
> >> @@ -2333,7 +2333,7 @@
> >> pStorage->pfnCompleted = pfnCompleted;
> >>
> >> /* Open the file. */
> >> - int rc = RTFileOpen(&pStorage->File, pszLocation, fOpen);
> >> + int rc = RTFileOpen(&pStorage->File, pszLocation, fOpen |
> >> RTFILE_O_NO_CACHE);
> >> if (RT_SUCCESS(rc))
> >> {
> >> *ppStorage = pStorage;
> >
> > This change does not make much sense. Better disable the host I/O
> > cache in the VM settings which is the correct strategy.
>
> That setting is only used to control disable host i/o for direct
> attach raw device. It doesn't apply to the disk files you opened. this
> is what I read from code. Correct me if i am wrong.
Yes, you are wrong :) If you disable the host cache for a storage
controller, all attached mediums will use asynchronous I/O and
will access the host medium with O_DIRECT. There are a few exceptions
(AFAIR for example NFS) but it should be visible in the VBox.log
file if the host cache is used or not.
Kind regards,
Frank
--
ORACLE Deutschland B.V. & Co. KG Dr.-Ing. Frank Mehnert
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