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

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Date:	Thu, 2 Jun 2011 21:02:33 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Ted Ts'o <tytso@....edu>, Dave Chinner <david@...morbit.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Query about DIO/AIO WRITE throttling and ext4 serialization

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:


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,
--
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