Re: Query about DIO/AIO WRITE throttling and ext4 serialization

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Jun 02, 2011 at 09:02:33PM -0400, Christoph Hellwig wrote:
> On Thu, Jun 02, 2011 at 08:54:03PM -0400, Vivek Goyal wrote:
> > Just wondering why ext4 and XFS behavior are different and which is a
> > more appropriate behavior. ext4 does not seem to be waiting for all
> > pending AIO/DIO to finish while XFS does.
> 
> They're both wrong.  Ext4 completely misses support in fsync or sync
> to catch pending unwrittent extent conversions, and thus fails to obey
> the data integrity guarante.  XFS is beeing rather stupid about the
> amount of synchronization it requires.  The untested patch below
> should help with avoiding the synchronization if you're purely doing
> overwrites:

Yes this patch helps. I have already laid out the file and doing
overwrites.

I throttled aio-stress in one cgroup to 1 byte/sec and edited another
file from other cgroup and did a "sync" and it completed.

Thanks
Vivek

> 
> 
> Index: xfs/fs/xfs/linux-2.6/xfs_aops.c
> ===================================================================
> --- xfs.orig/fs/xfs/linux-2.6/xfs_aops.c	2011-06-03 09:54:52.964337556 +0900
> +++ xfs/fs/xfs/linux-2.6/xfs_aops.c	2011-06-03 09:57:06.877674259 +0900
> @@ -270,7 +270,7 @@ xfs_finish_ioend_sync(
>   * (vs. incore size).
>   */
>  STATIC xfs_ioend_t *
> -xfs_alloc_ioend(
> +__xfs_alloc_ioend(
>  	struct inode		*inode,
>  	unsigned int		type)
>  {
> @@ -290,7 +290,6 @@ xfs_alloc_ioend(
>  	ioend->io_inode = inode;
>  	ioend->io_buffer_head = NULL;
>  	ioend->io_buffer_tail = NULL;
> -	atomic_inc(&XFS_I(ioend->io_inode)->i_iocount);
>  	ioend->io_offset = 0;
>  	ioend->io_size = 0;
>  	ioend->io_iocb = NULL;
> @@ -300,6 +299,18 @@ xfs_alloc_ioend(
>  	return ioend;
>  }
>  
> +STATIC xfs_ioend_t *
> +xfs_alloc_ioend(
> +	struct inode		*inode,
> +	unsigned int		type)
> +{
> +	struct xfs_ioend	*ioend;
> +
> +	ioend = __xfs_alloc_ioend(inode, type);
> +	atomic_inc(&XFS_I(ioend->io_inode)->i_iocount);
> +	return ioend;
> +}
> +
>  STATIC int
>  xfs_map_blocks(
>  	struct inode		*inode,
> @@ -1318,6 +1329,7 @@ xfs_end_io_direct_write(
>  	 */
>  	iocb->private = NULL;
>  
> +	atomic_inc(&XFS_I(ioend->io_inode)->i_iocount);
>  	ioend->io_offset = offset;
>  	ioend->io_size = size;
>  	if (private && size > 0)
> @@ -1354,7 +1366,7 @@ xfs_vm_direct_IO(
>  	ssize_t			ret;
>  
>  	if (rw & WRITE) {
> -		iocb->private = xfs_alloc_ioend(inode, IO_DIRECT);
> +		iocb->private = __xfs_alloc_ioend(inode, IO_DIRECT);
>  
>  		ret = __blockdev_direct_IO(rw, iocb, inode, bdev, iov,
>  					    offset, nr_segs,