Re XFS. Have you been watching BTRFS recently?
I will concede that despite the authors considering it production ready I won't be using it for my servers just yet. However, it's benchmarking on single disk benchmarks fairly similarly to XFS and in certain cases (multi-threaded performance) can be somewhat better. I haven't yet seen any benchmarks on larger disk arrays yet, eg 6+ disks, so no idea how it scales up. Basically what I have seen seems "competitive"
I don't have such hardware spare to benchmark, but I would be interested to hear from someone who benchmarks your RAID1+linear+XFS suggestion, especially if they have compared a cutting edge btrfs kernel on the same array?
One of the snags of md RAID1 vs RAID6 is the lack of checksumming in the event of bad blocks. (I'm not sure what actually happens when md scrubbing finds a bad sector with raid1..?). For low performance requirements I have become paranoid and been using RAID6 vs RAID10, filesystems with sector checksums seem attractive...
Regards
Ed W