Linux 4.4 Will Bring Clustered RAID1 & Journaled RAID5 Support
In his pull request he notes the two big ticket items are SUSE's clustered RAID1 support is nearly done and Facebook has contributed their initial journaled RAID5 support.
Shaohua Li of Facebook was working on the journaled RAID5 code. "This introduces a simple log for raid5. Data/parity writing to raid array first writes to the log, then write to raid array disks. If crash happens, we can recovery data from the log. This can speed up raid resync and fix write hole issue."
More information on the MD changes for Linux 4.4 can be found by looking at the comments to the changes in Git.