>for applications that do large runs of sequential i/o, the same logic
>applies as well, if the application isn't going to access the data multiple
>times and it uses i/os of at least 64k.  read ahead doesn't gain you all
>that much when you're doing large i/os, especially on modern disks that
>do read ahead into the cache in the disk.  in this case there's basically
>no difference to the disk driver between doing the i/o to filesystem cache
>pages vs. doing the i/o straight to the application, and doing it direct saves
>a memory-to-memory copy.

Just an FYI ... with mods to ftp for large window sizes and to use SGI
direct I/O, I was able to get 60 MB/sec (megabytes per sec) between two
OC-12 connected machines, disk to disk (they were multi-processor Origin
boxes in the same machine room, but I still thought that was pretty good).
Without direct I/O I think I topped out at 20 MB/sec.

--Ken