An update on the Linux NFS server problems I had: massively high i/o by process jbd2/xvda1-8.
The initial fix I settled on was perhaps a file system corruption issue. That didn't work actually, even though initially it looked like it did.
Then I thought it maybe a Linux kernel. But even that didn't permanently fix it.
Issue just appeared again: added one more php5-fpm server to the mix, an identical copy of another one, and bang problem came back.
My latest solution: I was using an lvm2 stripe, switched to an md stripe. Seems to work so far.
Update: didn't work, see http://www.advogato.org/person/berend/diary/401.html