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SubjectRe: [RFC 0/7] [RFC] cramfs: fake write support
arnd@arndb.de 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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