linux-ext4 - Re: [PATCH 04/12] ext4: Disable merging of uninitialized extents

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Date:	Mon, 28 Jan 2013 10:38:36 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 04/12] ext4: Disable merging of uninitialized extents

On Mon, Jan 28, 2013 at 07:02:55PM +0400, Dmitry Monakhov wrote:
> Actually this patch consists of two peaces
> 1) disable merging of uninitialized extents. (1 line change) I'm
> absolutely agree with it.

To be clear, that's this patch chunk (one line change not including
comments :-), right?

--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1579,11 +1576,13 @@ int
 ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent *ex1,
 				struct ext4_extent *ex2)
 {
 
 	/*
-	 * Make sure that either both extents are uninitialized, or
-	 * both are _not_.
+	 * Make sure that both extents are initialized. We don't merge
+	 * uninitialized extents so that we can be sure that end_io code has
+	 * the extent that was written properly split out and conversion to
+	 * initialized is trivial.
 	 */
-	if (ext4_ext_is_uninitialized(ex1) ^ ext4_ext_is_uninitialized(ex2))
+	if (ext4_ext_is_uninitialized(ex1) || ext4_ext_is_uninitialized(ex2))
 		return 0;
 

The one thing I'm a bit worried about is how much worse will extent
fragmentation be once we do this, but it's clear we need to strive for
correctness first.

							- Ted
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