Linux – fault tolerant file systems

I think you'll have to abandon the POSIX requirement, very few systems implement that - in fact even NFS doesn't really (think locks etc) and that has no redundancy.

Any system which uses synchronous replication is going to be glacially slow; any system which has asynchronous replication (or "eventual consistency") is going to violate POSIX rules and not behave like a "conventional" filesystem.

If it were me, I would be using GlusterFS. The current release is pretty solid and I know people at some very large installations in both the HPC and Internet space that are relying on it in their production systems. You can basically tailor it to your needs by laying out the components as you need them. Unlike Lustre, there are no dedicated metadata servers so central points of failure are minimized, and it's easier to scale the setup.

Unfortunately I don't think there's an easy way to meet your 75% criteria without throwing performance down the drain.

It does run on commodity hardware, however the performance really shines when using Infiniband interconnect. Fortunately the price of IB is really quite low these days.

You might want to check out the guys at Scalable Informatics and their Jackrabbit products as a solution. They support GlusterFS on their hardware, and the price of their solution certainly rivals the cost of putting something together from scratch.