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Commit 67a5da56 authored by Zheng Liu's avatar Zheng Liu Committed by Theodore Ts'o
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ext4: make the zero-out chunk size tunable 



Currently in ext4 the length of zero-out chunk is set to 7 file system
blocks.  But if an inode has uninitailized extents from using
fallocate to preallocate space, and the workload issues many random
writes, this can cause a fragmented extent tree that will
unnecessarily grow the extent tree.

So create a new sysfs tunable, extent_max_zeroout_kb, which controls
the maximum size where blocks will be zeroed out instead of creating a
new uninitialized extent.  The default of this has been sent to 32kb.

CC: Zach Brown <zab@zabbo.net>
CC: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: default avatarZheng Liu <wenqing.lz@taobao.com>
Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent 81370291

Documentation/ABI/testing/sysfs-fs-ext4

+13 −0
Original line number Diff line number Diff line
@@ -96,3 +96,16 @@ Contact: "Theodore Ts'o" <tytso@mit.edu> 
Description:

		The maximum number of megabytes the writeback code will

		try to write out before move on to another inode.



What:		/sys/fs/ext4/<disk>/extent_max_zeroout_kb

Date:		August 2012

Contact:	"Theodore Ts'o" <tytso@mit.edu>

Description:

		The maximum number of kilobytes which will be zeroed

		out in preference to creating a new uninitialized

		extent when manipulating an inode's extent tree.  Note

		that using a larger value will increase the

		variability of time necessary to complete a random

		write operation (since a 4k random write might turn

		into a much larger write due to the zeroout

		operation).

fs/ext4/ext4.h

+3 −0
Original line number Diff line number Diff line
@@ -1271,6 +1271,9 @@ struct ext4_sb_info { 
	unsigned long s_sectors_written_start;

	u64 s_kbytes_written;



	/* the size of zero-out chunk */

	unsigned int s_extent_max_zeroout_kb;



	unsigned int s_log_groups_per_flex;

	struct flex_groups *s_flex_groups;

fs/ext4/extents.c

+13 −12
Original line number Diff line number Diff line
@@ -3085,7 +3085,6 @@ out: 
	return err ? err : map->m_len;

}



#define EXT4_EXT_ZERO_LEN 7

/*

 * This function is called by ext4_ext_map_blocks() if someone tries to write

 * to an uninitialized extent. It may result in splitting the uninitialized
@@ -3111,13 +3110,14 @@ static int ext4_ext_convert_to_initialized(handle_t *handle, 
					   struct ext4_map_blocks *map,

					   struct ext4_ext_path *path)

{

	struct ext4_sb_info *sbi;

	struct ext4_extent_header *eh;

	struct ext4_map_blocks split_map;

	struct ext4_extent zero_ex;

	struct ext4_extent *ex;

	ext4_lblk_t ee_block, eof_block;

	unsigned int ee_len, depth;

	int allocated;

	int allocated, max_zeroout = 0;

	int err = 0;

	int split_flag = 0;
@@ -3125,6 +3125,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle, 
		"block %llu, max_blocks %u\n", inode->i_ino,

		(unsigned long long)map->m_lblk, map->m_len);



	sbi = EXT4_SB(inode->i_sb);

	eof_block = (inode->i_size + inode->i_sb->s_blocksize - 1) >>

		inode->i_sb->s_blocksize_bits;

	if (eof_block < map->m_lblk + map->m_len)
@@ -3224,9 +3225,12 @@ static int ext4_ext_convert_to_initialized(handle_t *handle, 
	 */

	split_flag |= ee_block + ee_len <= eof_block ? EXT4_EXT_MAY_ZEROOUT : 0;



	/* If extent has less than 2*EXT4_EXT_ZERO_LEN zerout directly */

	if (ee_len <= 2*EXT4_EXT_ZERO_LEN &&

	    (EXT4_EXT_MAY_ZEROOUT & split_flag)) {

	if (EXT4_EXT_MAY_ZEROOUT & split_flag)

		max_zeroout = sbi->s_extent_max_zeroout_kb >>

			inode->i_sb->s_blocksize_bits;



	/* If extent is less than s_max_zeroout_kb, zeroout directly */

	if (max_zeroout && (ee_len <= max_zeroout)) {

		err = ext4_ext_zeroout(inode, ex);

		if (err)

			goto out;
@@ -3250,9 +3254,8 @@ static int ext4_ext_convert_to_initialized(handle_t *handle, 
	split_map.m_lblk = map->m_lblk;

	split_map.m_len = map->m_len;



	if (allocated > map->m_len) {

		if (allocated <= EXT4_EXT_ZERO_LEN &&

		    (EXT4_EXT_MAY_ZEROOUT & split_flag)) {

	if (max_zeroout && (allocated > map->m_len)) {

		if (allocated <= max_zeroout) {

			/* case 3 */

			zero_ex.ee_block =

					 cpu_to_le32(map->m_lblk);
@@ -3264,9 +3267,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle, 
				goto out;

			split_map.m_lblk = map->m_lblk;

			split_map.m_len = allocated;

		} else if ((map->m_lblk - ee_block + map->m_len <

			   EXT4_EXT_ZERO_LEN) &&

			   (EXT4_EXT_MAY_ZEROOUT & split_flag)) {

		} else if (map->m_lblk - ee_block + map->m_len < max_zeroout) {

			/* case 2 */

			if (map->m_lblk != ee_block) {

				zero_ex.ee_block = ex->ee_block;

fs/ext4/super.c

+3 −0
Original line number Diff line number Diff line
@@ -2541,6 +2541,7 @@ EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs); 
EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);

EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);

EXT4_RW_ATTR_SBI_UI(max_writeback_mb_bump, s_max_writeback_mb_bump);

EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);

EXT4_ATTR(trigger_fs_error, 0200, NULL, trigger_test_error);



static struct attribute *ext4_attrs[] = {
@@ -2556,6 +2557,7 @@ static struct attribute *ext4_attrs[] = { 
	ATTR_LIST(mb_stream_req),

	ATTR_LIST(mb_group_prealloc),

	ATTR_LIST(max_writeback_mb_bump),

	ATTR_LIST(extent_max_zeroout_kb),

	ATTR_LIST(trigger_fs_error),

	NULL,

};
@@ -3756,6 +3758,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent) 


	sbi->s_stripe = ext4_get_stripe_size(sbi);

	sbi->s_max_writeback_mb_bump = 128;

	sbi->s_extent_max_zeroout_kb = 32;



	/*

	 * set up enough so that it can read an inode