Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/03/2013 12:44, Alex Bligh ha scritto:
> If this isn't true, we have a problem anyway with (e.g.) emulated
> devices which don't use O_DIRECT anyway.

Yes, though that would be a libxl bug, not a QEMU bug.

> And I had thought (I may be
> wrong) using O_DIRECT does not guarantee no read caching with NFS;
> O_DIRECT merely guarantees the page cache is not used under Linux
> and isn't defined under POSIX:
>  http://www.spinics.net/lists/linux-nfs/msg17472.html

Read caching on the server is fine, because it is the same server that
was used for the writes.

O_DIRECT bypasses the client's page cache, and that's enough for our
purposes.

> If it were just a write caching issue, we could use O_DSYNC instead of
> O_DIRECT, which would at least ensure the copy from userspace.

O_DSYNC is not necessary.  We do issue the appropriate fsync/fdatasync.
 What O_DSYNC does is add an implicit fdatasync after every write,
basically.

Paolo