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

[Nathaniel S. <nr...@le...>]

Albrecht Gebhardt wrote:

>On Thu, Nov 03, 2005 at 02:58:04AM -0800, Nathaniel Stahl wrote:
>  
>
>>Almost right.  You need to populate the View so that it refers to the 
>>files in the read/only filesystem.  For files in the View: directories 
>>represent themselves, data files contain a path to the host file that 
>>backs them (can be newline terminated or not) relative to the lookup 
>>root (root=) mount parameter, other special files (symbolic links, 
>>fifos, device nodes) just represent themselves.
>>
>>
>>    
>>
<snip>

>ok, I'll try it. 
>
>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?
>
>Thanks for the quick answer
>
>Albrecht
>
>
>  
>

rw-tree should see new files that are created in the View.  It will also 
notice if a file is removed through the View directly and not through 
MapFS...  but you might get an error the first time you try to access 
the newly removed file.

Don't recreate all the View files... you'll invalidate the current MapFS 
files because the View inodes will change.  Just create files that are 
new and not in the View already.  If a file is currently open, MapFS 
will properly hold the reference open until it closes... but dcache 
entries may remain until the kernel decides to shrink the dcache or you 
try a lookup on a name that was unlinked and recreated.

warning: If a file in MapFS refers to a file on a writable filesystem, 
MapFS will remove it if you remove the file through MapFS (it thinks 
you're manipulating a potentially old Overlay).

NFS support is still a little shakey.  It's what I'm working on 
polishing now.  NFS is kinda the naughtiest filesystem I've found so far 
in terms of what it does under the covers.  It should mostly work.

-Nate