Re: [libvirt] [PATCH 7/8] save: add virDirectFd wrapper type
On Tue, Jul 19, 2011 at 10:08:21AM -0600, Eric Blake wrote:
On 07/19/2011 10:06 AM, Daniel P. Berrange wrote:
>
>The motivation for using O_DIRECT is that allowing pollution of the
>host cache causes stability problems for the host as a whole. As
>such IMHO, apps would likely want an error back if O_DIRECT cannot
>be supported,
>
>NB, even some Linux filesystems can't do O_DIRECT, so this isn't an
>obscure mingw32 issue.
Conversely, open() on Linux silently ignores unknown flags - so if
you are using a really old kernel but newer glibc headers, then
O_DIRECT is non-zero and open() succeeds, but you _don't_ get direct
I/O.
If O_DIRECT is 0, then it is pretty easy to diagnose that the
request is unsupported. But if O_DIRECT is non-zero, then how do I
tell whether the open(O_DIRECT) really meant that I have direct I/O,
or whether it was a nice hint but still ignored and I'm still
polluting the file system cache?
Hmm, I could have sworn we've seen QEMU itself fail to start when
requesting O_DIRECT on say, tmpfs. Perhaps open() isn't failing,
but rather read/write fail ?
Daniel
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