Re: [RFC 0/7] [RFC] cramfs: fake write support

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arnd@xxxxxxxx wrote:
Many existing systems currently use unionfs or aufs for this
purpose, by overlaying a tmpfs over a read-only file
system like cramfs, squashfs or iso9660. IMHO, it would
be a much nicer solution to not require unionfs for a simple
case like this, but rather have support for it in the file
system. If people find this useful, we can do the same in
other read-only file system.

I think it's a good idea, and I have been thinking about adding something similar to Squashfs for a quite a while (when I get time).

Comments?

Patch 2 ([RFC 2/7] cramfs: create unique inode numbers) changes the inode number to be based on the dentry location rather than the file location. This is a user-visible change, not only do empty directories, char, block, pipe, and sockets get real inode numbers rather than 1 (a good thing IMHO), but files that were hard-linked (in the original source directory) now get different inode numbers. Obviously cramfs has never properly supported hard links, but the duplicate file check in cramfs did ensure hard linked files got the same inode number.

This change in behaviour may break some existing users of cramfs filesystems. It may be worth sending the RFC and patches etc. to the new linux-embedded mailing list to get some feedback from the embedded folks who use cramfs.

Phillip


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