Re: [Qemu-devel] [PATCH 1/2] hw/9pfs: Add new virtfs option cache=writethrough to skip host page cache

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wed, 12 Oct 2011 10:55:21 +0100, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Mon, Oct 10, 2011 at 10:06 AM, Aneesh Kumar K.V
> <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > diff --git a/hw/9pfs/virtio-9p-handle.c b/hw/9pfs/virtio-9p-handle.c
> > index 5c8b5ed..441a37f 100644
> > --- a/hw/9pfs/virtio-9p-handle.c
> > +++ b/hw/9pfs/virtio-9p-handle.c
> > @@ -202,6 +202,15 @@ static ssize_t handle_pwritev(FsContext *ctx, int fd, const struct iovec *iov,
> >         return writev(fd, iov, iovcnt);
> 
> The sync_file_range(2) call below is dead-code since we'll return
> immediately after writev(2) completes.  The writev(2) return value
> needs to be saved temporarily and then returned after
> sync_file_range(2).

Missed that. Will fix in the next update

> 
> >     }
> >  #endif
> > +    if (ctx->cache_flags & V9FS_WRITETHROUGH_CACHE) {
> 
> -drive cache=writethrough means something different from 9pfs
> "writethrough".  This is confusing so I wonder if there is a better
> name like immediate write-out.
> 

cache=immediate-write-out ? 

> > +        /*
> > +         * Initiate a writeback. This is not a data integrity sync.
> > +         * We want to ensure that we don't leave dirty pages in the cache
> > +         * after write when cache=writethrough is sepcified.
> > +         */
> > +        sync_file_range(fd, offset, 0,
> > +                        SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE);
> > +    }
> 
> I'm not sure whether SYNC_FILE_RANGE_WAIT_BEFORE is necessary.  As a
> best-effort mechanism just SYNC_FILE_RANGE_WRITE does the job although
> a client that rapidly rewrites may be able to leave dirty pages in the
> host page cache

Shouldn't we prevent this ? That is the reason for me to use that
WAIT_BEFORE ?

>.  SYNC_FILE_RANGE_WAIT_BEFORE ensures that dirty pages
> get written out but it is no longer asynchronous because it blocks.

-aneesh