Messages in this thread | | | Date | Tue, 16 Mar 2004 19:04:13 +0100 | From | Jörn Engel <> | Subject | Re: unionfs |
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On Tue, 16 March 2004 18:31:47 +0100, Jörn Engel wrote: > On Tue, 16 March 2004 12:04:30 -0400, Horst von Brand wrote: > > > > What happens if I mount the live 2.6.4 kernel source over a CD containing > > 2.5.30? What happens to identical files, files that moved, changed files, > > deleted files? Pray tell, how does the kernel find out which is which? > > What happens if I write to /dev/hda while having my rootfs /dev/hda1? > Bad things, damn right. But why would anyone do that?
There is really no point to this discussion, as it looks like a big misunderstanding. Maybe you object less if you see the design:
Variant 1 (just a single filesystem):
- Introduce a new variant of links, which I call COW. - COWs can only link to hidden inodes. - Hidden inodes cannot be accessed directly. - COWs look like regular files to userspace. - Read access to COWs goes to the hidden inode. - Write access to COWs copies the hidden inode before writing to it. - Copying file1 to file2 does four things: - Create a new hidden inode. - Move data from file1 to hidden inode. - Turn file1 into COW and link to hidden inode. - Create COW for file2 and link to hidden inode.
There are some more special cases, but this is basically it. So let's use the stuff a little:
$ cp -cr dir1 dir2
Behaves similar to 'cp -lr', but creates COWs instead of hard links. Can take a few seconds to create the directories, but not minutes.
$ vi dir2/bunch*of*files
Writing to those files makes a real copy for each. dir1/* remains unaffected, no matter how careless you are. We've made it foolproof, so the universe has to create greater fools again, right?
$ rm -rf dir2
Scraps one of the copies along with all modifications. dir1/* remains unaffected gain.
$ cp -cr /fs1/dir1 /fs2/dir2
Fails, since links between different filesystems don't work.
Variant 2 (across multiple filesystems now):
- COWs contain a filesystem identifier as well. - Accessing COWs linking to unavaillable filesystems returns -E... Alternatively: - Mounting such an fs fails, unless all links work.
Usage is as above.
$ mkfs /dev/fs2 $ mount /dev/fs2 /fs2 $ cp -cr /fs1 /fs2
Creates an identical copy of one filesystem on another one. fs2 has to support COWs and fs2 has to be RO or support COWs. A rw-fs mounted ro means trouble, as you know.
Maybe I'm just stupid and missed some important detail, but this design looks like it can solve a bunch of problems. Do you still think, it is useless?
Jörn
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