Well, I decided to do a quick rebuild of my fileserver's array since my new drives came in. I have used Ext3, ReiserFS 3.6 as well as XFS, so I figured I would try Reiser4. Ext3 had good performance for large files, but doesn't seem to do so well in folders with a few tens of thousands of small files. Reiser3 had good all around performance, especially with smaller files, although when I had some hardware issues reiserfschk seemed to love deleting files. I loved the agressive caching of XFS (make sure you have a UPS!) but it really really really *stinks* at deleting files.
So, with that being said here is my initial reaction to Resier4.
Hans is my new personal Deity. Reiser4 is much faster then Reiser 3.6 when reading large files (it seems to be a lot closer to Ext2/3 for reading files that are more than a couple gigabytes in size). It seems to be at least as fast as XFS for streaming large files off disk, and it didn't choke at my "test" folder with roughly 1.2 million files in it (that dir is a great way to choke a lot of filesystems).
Here is what I am running for hardware & software:
Intel 7505VB2 Xeon mobo
2 x 3.06/512 Xeons
2GB RAM (2x1GB)
3Ware Escalade 8506-12 with 8x250GB SATA, 2x300GB SATA, 2x160GB SATA
3Ware Escalade 8506-8 with 8x200GB SATA
3Ware Escalade 7500-4 with 3x200GB IDE
Linux kernel 2.6.9-RC1-mm2
My arrays compose mainly of larger files, a few to a directory (1-8GB in size each). The small files are all tiny, generally 500 bytes-32 Kbytes.