[ale] Copy-on-write file?

[JK]

Hi everyone,

ddrescue kicks a*s -- it imaged all but about 20MB of my 80GB failed
HFS partition successfully.  I then made a copy of the image so as to
have a backup in case I hosed the original (via fsck or simple
dumbness on my part).  Making the backup took about 5 hours, which
kinda sucked; and if I *do* need to revert to the backup image,
copying for another 5 hours is not an appealing prospect.  What I'd
really like is a way to tell the Linux kernel: "Please treat this
file as copy-on-write -- if I write something to it, allocate a new
on-disk block to hold the written data, splice that block into the
file, and leave the original alone."  That way, going back to the
clean image would merely be a matter of throwing away the COW blocks,
instead of waiting another 5 hours to copy my backup image.

Surely such magic is already available somehow? I know there are some
distros that implement "writable" CDROM filesystems in a similar
way, by using specialized COW filesystems, but that isn't exactly
what I want.  I want to either
(a) set up COW for a single file on an EXT3 FS, or else
(b) mount the disk image as an HFS+ volume, in such a way that writes
will not alter the image itself.

(BTW I found the ext3cow FS, which would work, but something less
intrusive would be awfully nice.)

TIA,

-- JK


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