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Re: [libvirt] [PATCH 7/8] save: add virDirectFd wrapper type

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?

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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