Now, we found ldiskfs(Ext4) directory creation is much slower than file creation.

Under RHEL7 with ldiskfs, we got following results.

1. ./mdtest -d /mnt/test/ -n 100000 -i 3

Directory creation: 52299.122 ops/second
File creation: 106569 ops/second

As we can see, directory creation performance is much slower than file creation, after doing some profiling, we found cost differences come from following calls:

Directory creation: 111276.454 ops/second
File creation: 114920.338 ops/second

As we can see, with inline_data enabled, directory creation performance is same as file creation.

We also found inline_data feature not only help directory creation but also help
directory reading, consider following case:

1. creating 100M directories under test directory and then run 'time ls -R ./test'

time cost reduce from 2m25s to 30s, huge differences! this is because inline_data will reduce an extra block allocation which also speed read performance as we don't have to do an extra block IO.

In generally, Inline_data could improve performance and reduce space allocation which is also good.

As now, inline_data is include in RHEL7 and there seems some conflicts with Lustre dirdata feature, but inline_data deserve us an eye, adding it to lustre will give us some improvement thought we still confirm it under lustre.