Re: [libvirt] PATCH: Disable QEMU drive caching

[Anthony Liguori <anthony codemonkey ws>]

Anthony Liguori wrote:
> Mark McLoughlin wrote:
>
> And let's take a step back too.  If people are really concerned about 
> this point, let's introduce a sync=on option that opens the image with 
> O_SYNC.  This will effectively make the cache write-through without 
> the baggage associated with O_DIRECT.

I'm starting to slowly convince myself we should always open files with 
O_SYNC.  Barriers should just force ordering within the thread pool.  
posix-aio has no interface for this but we could create one with our own 
thread pool implementation.

Ryan: could you give the following patch a perf-run so we can see how 
this would effect us?

Thanks,

Anthony Liguori

> While I object to libvirt always setting cache=off, I think sync=on 
> for IDE and SCSI may be reasonable (you don't want it for virtio-blk 
> once we implement proper barriers with fdatasync I think).
>
> Regards,
>
> Anthony Liguori
>
> > Cheers,
> > Mark.
> >
> >   

diff --git a/block-raw-posix.c b/block-raw-posix.c
index c0404fa..0d8ffdb 100644
--- a/block-raw-posix.c
+++ b/block-raw-posix.c
@@ -121,7 +121,7 @@ static int raw_open(BlockDriverState *bs, const char *filename, int flags)
 
     s->lseek_err_cnt = 0;
 
-    open_flags = O_BINARY;
+    open_flags = O_BINARY | O_SYNC;
     if ((flags & BDRV_O_ACCESS) == O_RDWR) {
         open_flags |= O_RDWR;
     } else {