very large ramdisk setup problems

I'm using Red Hat Enterprise 4, with a 2.6.12 kernel.

I'm trying to set up a very large RAM disk -- 2 GB, out of 8 GB available. The application I'm working on has high numbers of file writes. We find that when we put temporary files on ramdisk and operate upon them before writing permanent files we get better throughput.

We are looking at RAMDisk as a way to avoid having those temporary files flushed from cache to disk before we get to operate upon them (which results in re-writing the entire file -- the final file does go to regular disk).

I've looked at a variety of options...

1. ramfs -- to primitive. I need reporting of consumption. 2. tmpfs -- seems to work, but it's swappable. We don't want the temp files swapped, because we're I-O bound. Also, performance appears to suffer if I use tmpfs instead of ext2. 3. ext2, reiserfs. For both of these file systems mkfs fails if the ramdisk file system attempted is greater than 512 MB. If the file system is limited to 512 MB they work. (note, I tried reiserfs after ext2 failed -- I know it journals and that's a waste here)

Questions... 1. Short of disabling swapping entirely, is there any way to keep tmpfs from being swapped? (Since it's using vm, it would seem not) 2. If I use tmpfs do I need to have the ramdisksize option specified as a boot parameter? It would seem not... 3. There seems to be some 512MB limit on ramdisk. Why? I can find references to others running into it but no solutions proposed other than to use tmpfs. How can I get around this problem? 4. If I build a ramdisk in ext2 or some other regular file system will it swap?

Thanks Matt