In fact, I want to implement a system that include TFTF, DHCP, and NFS server and that can boot diskless clients by PXE. Currently, multiple clients can be boot from a read-only images stored on the NFS simultaneously and all writes cache on the RAM. But, I want the written data are stored on a directory which is specifically for a client instead of the RAM. There is a document, the aufs root file system on usb flash, which can redirect all writes to the RAM, I have read, but I don't have a good idea how to modify it for. Can anyone suggest that how to redirect all writes into a specific directory?

Wile guess, I might be tempted to use something like a usb casper persistence file.

Depending on the system booted, it may be able to use a save file to almost any location. Puppy and maybe DSL, slitaz and others may have ways to do that when booted toram.

Might be possible to run a slightly different deal like you say where a fuse filesystem is attached to nfs or iscsi.

Might be possible to link iso and use some fuse or aufs to some unique location on boot.

Guess we'd need to know more about how this base system has options for using or we'd need to go on to a combined filesystem.