[PATCH] tunable ext3 commit interval

The patch from Stephen Tweedie allows users to modify the journal
commit interval for the ext3 filesystem.

The commit interval is normally five seconds.  For portable computers
with spun-down drives it is advantageous to be able to increase the
commit interval.

There may also be advantages in decreasing the commit interval for
specialised applications such as heavily-loaded NFS servers which are
using synchronous exports.

The laptop users will also need to increase the pdflush periodic
writeback interval (/proc/sys/vm/dirty_writeback_centisecs), because
the `kupdate' activity also forces a commit.

To specify the commit interval, use

	mount -o commit=30 /dev/hda1 /mnt/whatever

or
	mount -o remount,commit=30 /dev/hda1

The commit interval is specified in units of seconds.

fs/ext3/super.c

@@ -696,6 +696,11 @@ static int parse_options (char * options, unsigned long * sb_block,
 				*mount_options &= ~EXT3_MOUNT_DATA_FLAGS;
 				*mount_options |= data_opt;
 			}
+		} else if (!strcmp (this_char, "commit")) {
+			unsigned long v;
+			if (want_numeric(value, "commit", &v))
+				return 0;
+			sbi->s_commit_interval = (HZ * v);
 		} else {
 			printk (KERN_ERR 
 				"EXT3-fs: Unrecognized mount option %s\n",
@@ -1260,6 +1265,22 @@ static int ext3_fill_super (struct super_block *sb, void *data, int silent)
 	return -EINVAL;
 }
 
+/*
+ * Setup any per-fs journal parameters now.  We'll do this both on
+ * initial mount, once the journal has been initialised but before we've
+ * done any recovery; and again on any subsequent remount. 
+ */
+static void ext3_init_journal_params(struct ext3_sb_info *sbi, 
+				     journal_t *journal)
+{
+	if (sbi->s_commit_interval)
+		journal->j_commit_interval = sbi->s_commit_interval;
+	/* We could also set up an ext3-specific default for the commit
+	 * interval here, but for now we'll just fall back to the jbd
+	 * default. */
+}
+
+
 static journal_t *ext3_get_journal(struct super_block *sb, int journal_inum)
 {
 	struct inode *journal_inode;
@@ -1294,7 +1315,7 @@ static journal_t *ext3_get_journal(struct super_block *sb, int journal_inum)
 		printk(KERN_ERR "EXT3-fs: Could not load journal inode\n");
 		iput(journal_inode);
 	}
-	
+	ext3_init_journal_params(EXT3_SB(sb), journal);
 	return journal;
 }
 
@@ -1371,6 +1392,7 @@ static journal_t *ext3_get_dev_journal(struct super_block *sb,
 		goto out_journal;
 	}
 	EXT3_SB(sb)->journal_bdev = bdev;
+	ext3_init_journal_params(EXT3_SB(sb), journal);
 	return journal;
 out_journal:
 	journal_destroy(journal);
@@ -1667,6 +1689,8 @@ int ext3_remount (struct super_block * sb, int * flags, char * data)
 
 	es = sbi->s_es;
 
+	ext3_init_journal_params(sbi, sbi->s_journal);
+	
 	if ((*flags & MS_RDONLY) != (sb->s_flags & MS_RDONLY)) {
 		if (sbi->s_mount_opt & EXT3_MOUNT_ABORT)
 			return -EROFS;