[ILUG] Merging directories transparently

[Brian Foster]

  | Date: Mon, 25 Mar 2002 02:53:25 +0000 (GMT)
  | From: Paul Jakma <paul at clubi.ie>
  | 
  | On Sun, 24 Mar 2002, David Neary wrote:
  | > That hides all (0) files in /tmp/test, though, rather than
  | > merging the contents of the two directories.
  | 
  | indeed it does. but i dont think there is any way (at this time) of
  | merging files on different volumes into one dir on linux, be it by
  | nfs or any other means.

 AFAIK, this is correct.  I myself have doubts it ever will be
 possible, as there may be some hairy issues to resolve --- but
 I long ago stopped trying to guess what new "madness"  ;-)
 Linux kernel developers would try ....

 some of the problems include, but may not be limited to:

  ( we want to, in effect, merge the namespaces of "directory"
   FULL and "directory" ROOM, nominally on different FS/volumes. )

 1. both dirs contain a file called A.  what happens?

 2. where are new files which are created written (i.e., which FS)?

 3. naïvely, the st_dev --- see stat(2) --- will differ from file
    to file as the merged directory is scanned.  albeit this can
    happen anyways (mount points), it's often(?) not handled too
    well by programs which care.

 file deletion may also be an issue, esp.(?) depending on the
 answer to `1' (duplicate names).  I doubt subdirs of FULL or
 ROOM are a problem, but I haven't thought about this ....

 in addition, at least one of FULL or ROOM has an independent
 existence outside the merged dir; i.e., can be changed, moved,
 or removed by _not_ going through the merged dir.  I suspect
 how much of an issue this is depends on the exact semantics.

  | afraid all the files will have to go on one volumes. either that or
  | you'll need to make use of the mkdir command. :)

 yes, the issue with symlinks is they are a b***er to maintain;
 esp. as files are created in, or removed from, the FULL dir.

 I myself would consider simply moving the files from FULL to
 ROOM --- this not only produces an easily-maintained merged
 dir, it also frees up space on FULL's FS/volume.

cheers!
	-blf-
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