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From: Herbert P÷tzl (herbert_at_13thfloor.at)
Date: Mon 18 Aug 2003 - 18:38:33 BST


Hi Everyone!

I'm glad to announce that Alex and I agreed upon
a quasi standard how to store context information
in filesystem inodes ...

What is your benefit?

 - the binary filesystem data will be compatible
   so changing from one implementation to the other
   should work (at least regarding this information)
 
 - testing and quality assurance should be improved
   because we only have to verify one implementation
   
 - tools will be available to read/modify/write this
   on disk ctx data, hopefully also backup solutions
   will be possible ...

Why should we do it at all?

 - there are some reasons for 'tagging' an inode
   with a 'context tag', like security issues or
   statistical purposes, but the major reason is
   a quota solution, which gives per vserver user
   and group quota, as well as some kind of easily
   configurable disk limit for each context on
   shared partitions ...

What was agreed upon?

 - there are three ways to store the 16bit context
   information within a on disk inode structure
   namely UID32/GID32, UID32/GID16, and UID24/GID24
   
 - UID32/GID32 or EXTERNAL format uses, up to now
   unused space within the disk inode to store the
   context information, this is currently only
   defined for ext2/ext3 but will be also defined
   for xfs, reiserfs, and jfs as soon as possible.
   you'll have full 32bit uid/gid values here ...
   
 - UID32/GID16 (works on all 32bit U/GID FSs)
   this format uses the upper half of the group id
   to store the context information. this is done
   transparently, so you'll never notice, except if
   you change the format without file conversion.
   it also means that you only have 16bits for gid
   so a maximum of 65536 groups ...
   
 - UID24/GID24 (works on all 32bit U/GID FSs)
   the format uses the upper quarter of user and
   group id to store the context information, again
   transparently. you'll end up with 16 million
   user and group ids, which should suffice for the
   majority of applications ...

 - but most important, the format you choose will
   have no influence on the features provided by
   the patches, so it's your preference ...
   
enjoy,
Herbert


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