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: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5a97e590692d..0423e2e7f615 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -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