As far as things have been described, you cannot do pure network boot without local storage. You need to hit the SD card first, and have it do the netboot.
You can have the best of both worlds: SD card does netboot and the rest of the space is scratch storage.
Because of the design of the R-Pi you have to have the SD card for booting. It loads the binary off the FAT partition and then passes control to the next partition (EXT2 (with/without knobs on) to get the boot image. This one can be a netboot. So it then sucks the remaining binary over cat5.
Your application then NFS/Samba's its data from some external storage, if necessary, and stores it temporarily on the remaining SD. When it has finished it returns the answers to the more respectable host and deletes the scratch.
Depending on how you want to do it you could either have scratch in a separate directory or give it a whole new partition. Presumably the R-Pi won't get confused by 3 partitions (?).
Three partitions is fine.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
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