Re: [mapfs-user] how to make a read-only fs writable?

[Albrecht G. <alb...@un...>]

On Thu, Nov 03, 2005 at 04:29:58AM -0800, Nathaniel Stahl wrote:
> Albrecht Gebhardt wrote:
>=20
> >On Thu, Nov 03, 2005 at 02:58:04AM -0800, Nathaniel Stahl wrote:
> >=20
> ...
>=20
> >ok, I'll try it.=20
> >
> >Does it mean that the rw-tree will not automatically see files which
> >appear after the mapfs mount in the ro-tree? (this can not occur for a
> >cdrom but e.g. is possible for a readonly nfs mount)
> >
> >Then I have to re-run the populate-view script?
> >
> ...
>=20
> rw-tree should see new files that are created in the View.  It will also=
=20
> notice if a file is removed through the View directly and not through=20
> MapFS...  but you might get an error the first time you try to access=20
> the newly removed file.
>=20
> Don't recreate all the View files... you'll invalidate the current MapFS=
=20
> files because the View inodes will change.  Just create files that are=20
> new and not in the View already.  If a file is currently open, MapFS=20
> will properly hold the reference open until it closes... but dcache=20
> entries may remain until the kernel decides to shrink the dcache or you=
=20
> try a lookup on a name that was unlinked and recreated.
>=20
> warning: If a file in MapFS refers to a file on a writable filesystem,=20
> MapFS will remove it if you remove the file through MapFS (it thinks=20
> you're manipulating a potentially old Overlay).
>=20
> NFS support is still a little shakey.  It's what I'm working on=20
> polishing now.  NFS is kinda the naughtiest filesystem I've found so far=
=20
> in terms of what it does under the covers.  It should mostly work.

Currently I'm using overlay filesystems for readonly cdrom and network
installations (cloop, NFS and mostly AFS).  I use it only for the /etc
subtree, currently I use mulafs after I tried unionfs (with many
kernel Oopses) and minifo_fs (still with some Ooopses) without
success. mulafs is more or less stable (only one unnecessary init
script crahses after a kernel Oops). But now I'll give a try to mapfs
=2E..

Albrecht

PS:
I prepared an ubuntu/debian package for mapfs, available at
http://ubuntu.uni-klu.ac.at/

The module packages are for our local patched version of breezy kernel
2.6.12, but the mapfs-source package can be used with module-assistent
to build binary packages for other kernels (I guess only 2.6 will
work, I modified the Makefile only for this case). Currently I have
also problems with th k7 version, it links the _read_lock,
_read_unlock, _spin_unlock and _spin_lock symbols which are only
exported in my k7-smp and 686-smp kernels. The 686 module works fine.

To rebuild all from scratch grab the source package components
mapfs-source* from=20
http://ubuntu.uni-klu.ac.at/ubuntu.uniklu/dists/hoary/uniklu/source/
and use dpkg-source, debuild and module-assistant.

>=20
> -Nate
>=20

--=20
// Albrecht Gebhardt          Tel.: (++43 463) 2700/3118
// Institut fuer Mathematik   Fax : (++43 463) 2700/3198
// Universitaet Klagenfurt    mailto:alb...@un...
// Universitaetsstr. 65       http://www.math.uni-klu.ac.at/~agebhard
// A-9020 Klagenfurt, Austria
// GPG PK: http://www.math.uni-klu.ac.at/~agebhard/agebhard.asc
// GPG FP: F46F 656E E83C 9323 CE30  FF8F 9DBA D1A3 B55A 78A6