tree 525f256d46cfac4be0b0acd90cc2bad5fcdb1b77
parent 81370291722ac1e0ec95234a0ea91a5bc76b6185
author Zheng Liu <wenqing.lz@taobao.com> 1345211657 -0400
committer Theodore Ts'o <tytso@mit.edu> 1345211657 -0400

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>