Messages in this thread | | | From | Jan Blunck <> | Subject | Re: [RFC] Union mounts/writable overlays design | Date | Thu, 1 Oct 2009 19:55:14 +0200 |
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Am 01.10.2009 um 19:15 schrieb Valerie Aurora <vaurora@redhat.com>:
> On Thu, Oct 01, 2009 at 10:38:27AM -0500, kevin granade wrote: >> Wow, amazingly thorough writeup, was a very interesting read and >> I'm looking >> forward to trying this out. >> >> Examples >>> ======== >>> >>> What happens when I... >>> >>> - creat() /newfile -> creates on top layer >>> - unlink() /oldfile -> creates a whiteout on top layer >>> - Edit /existingfile -> copies up to top layer at open(O_WR) time >>> - truncate /existingfile -> copies up to top layer + N bytes if >>> specified >>> - touch()/chmod()/chown()/etc. -> copies up to top layer >>> - mkdir() /newdir -> creates on top layer >>> - rmdir() /olddir -> creates a whiteout on top layer >>> - mkdir() /olddir after above -> creates on top layer w/ opaque flag >>> - readdir() /shareddir -> copies up entries from bottom layer as >>> fallthrus >>> - link() /oldfile /newlink -> copies up /oldfile, creates /newlink >>> on top >>> layer >>> - symlink() /oldfile /symlink -> nothing special >>> - rename() /oldfile /newfile -> copies up /oldfile to /newfile on >>> top layer >>> >> >> Minor quibble here, rename should also whiteout /oldfile, of course >> you have >> it explained correctly in the detailed description of rename() below. >> Or am I misunderstanding and the above is what it does now and the >> detailed >> description is what it will do once implemented properly? > > Hi Kevin, > > You are correct, it whiteouts the original name. Thanks for pointing > that out! > >>> Non-features >>> ------------ >>> >>> Features we do not currently plan to support as part of writable >>> overlays: >>> >>> Online upgrade: E.g., installing software on a file system NFS >>> exported to clients while the clients are still up and running. >>> Allowing the read-only bottom layer to change while the writable >>> overlay file system is mounted invalidates our locking strategy. >>> >> >> So as long as the RO filesystem is NOT mounted as part of an >> overlay, you >> could modify it and then re-construct the previous overlay and >> things will >> work as expected? >> For example could one create a hard drive over CD overlay, then >> periodically >> (requiring a reboot probably) replace the underlying CD with a new >> version >> and automagically have new versions of software available? >> ( obviously >> there are additional complexities in packaging to make this work, >> but having >> support in the kernel is the first step. ) > > This could theoretically work, but the main problem is resolving > differences between files (always the big problem in upgrade). Say > you have /etc/passwd, you add a new user and write to it on the top > layer, and then the next upgrade adds a new user to the read-only > version. You're not going to see the new user. >
No. Scripts that come with updated packages still need to run on the union. Otherwise this is just asking for problems. Probably you could come up with a clever merger if the update and the base image is still available.
>> One last thing, I don't see this in either the "features" or the >> "non-features". Will there be a way to "revert" a file to the RO >> version >> once it has been copied up, either by just removing the overlay >> entry or by >> somehow forcing the open of the underlying file when it has an >> overlay? Now >> that I think of it, one could just mount the underlying filesystem >> elsewhere >> and copy the file, but I'd still be interested to know if there is >> any >> desire to provide the more "direct" operation. > > I think that people are calling this "punch-through." I don't see a > problem with it, other than slightly more kernel support. > > -VAL
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