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Date:	Tue, 18 Sep 2012 01:00:30 +0400
From:	Andrey Sidorov <qrxd43@...orola.com>
To:	linux-ext4@...r.kernel.org
Subject: [PATCH RFC 1/2] ext4: speed-up truncate/unlink

Do not iterate over data blocks scanning for bh's to forget as they're
never exist. This improves time taken by unlink / truncate syscall.
Tested by continuously truncating file that is being written by dd.
Another test is rm -rf of linux tree while tar unpacks it. With
ordered data mode condition unlikely(!tbh) was always met in
ext4_free_blocks. With journal data mode tbh was found only few times,
so optimisation is also possible.

Unlinking fallocated 60G file after doing sync && echo 3 >
/proc/sys/vm/drop_caches && time rm --help

X86 before (linux 3.6-rc4):
# time rm -f test1
real    0m2.710s
user    0m0.000s
sys     0m1.530s

X86 after:
# time rm -f test1
real    0m0.644s
user    0m0.003s
sys     0m0.060s

MIPS before (linux 2.6.37):
# time rm -f test1
real    0m 4.93s
user    0m 0.00s
sys     0m 4.61s

MIPS after:
# time rm -f test1
real    0m 0.16s
user    0m 0.00s
sys     0m 0.06s

---
 fs/ext4/extents.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Index: linux-3.6-rc6/fs/ext4/extents.c
===================================================================
--- linux-3.6-rc6.orig/fs/ext4/extents.c	2012-09-17 12:08:48.842901966 -0400
+++ linux-3.6-rc6/fs/ext4/extents.c	2012-09-17 12:09:43.403630792 -0400
@@ -2274,10 +2274,13 @@ static int ext4_remove_blocks(handle_t *
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	unsigned short ee_len =  ext4_ext_get_actual_len(ex);
 	ext4_fsblk_t pblk;
-	int flags = EXT4_FREE_BLOCKS_FORGET;
+	int flags = 0;

 	if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
-		flags |= EXT4_FREE_BLOCKS_METADATA;
+		flags |= EXT4_FREE_BLOCKS_METADATA | EXT4_FREE_BLOCKS_FORGET;
+	else if (ext4_should_journal_data(inode))
+		flags |= EXT4_FREE_BLOCKS_FORGET;
+
 	/*
 	 * For bigalloc file systems, we never free a partial cluster
 	 * at the beginning of the extent.  Instead, we make a note
--
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