Ext4 encryption

> If your CPU doesn't have AESNI or equivalent, it uses lots of CPU when doing an I/O heavy operation (like a full backup). The performance difference when going from a CPU without AESNI to a CPU with AESNI is noticeable.

Ouch, yes, the difference is huge. At one point we had a server which was going dog-slow, while an apparently identical server next to it had no problems. After much searching it turned out that for some reason on this server the aesni_intel module was not getting loaded. An extra line in /etc/modules and a reboot later everything back to normal.

If the goal of this patch it to make as much of the filesystem usable without invoking any encryption then it explains some of the choices, but it's definitely a feature aimed at embedded processors.