Re: lazy old guy asks question

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

this is a contribution to the discussion what a plain disk image
records intrinsically and what many backup programs record only on
special request or don't record at all.

Andy Smith wrote:
> > Though if the goal is disk imaging there are some complications with
> > tar:
> > - By default doesn't preserve extended attributes unless you make use of
> >   some quite obscure options that you'll have to look up in the
> >   documentation: --xattrs --xattrs-include='*.*' (both required, no I
> >   can never remember this)

alain williams wrote:
> I have been playing with it to see how it recorded POSIX ACLs. You get lines
> like the one below; I have not worked out what it means
> 
> system.posix_acl_access=0sAgAAAAEABwD/////AgABAG4AAAACAAEAdQAAAAQAAQD/////EA
> ABAP////8gAAAA/////w==

That's the internal representation of ACL as xattr in the "system."
namespace of xattr.
AFAIK it is not necessarily portable between different fileystem types
or operating systems.

Besides xattr (getfattr(1)/setfattr(1)) and ACL (getfacl(1)/setfacl(1))
there are also the filesystem attributes as of lsattr(1)/chattr(1).
Among them are the 'A' flag for not updating atime, 'i' for immutable
files, 's' for zeroizing content on deletion of the file.
See man 1 chattr for a longer list.
Recording them is not too difficult. But restoring is tricky, because
some of them get set only by the filesystem driver under special
conditions. The backup program has to offer some filtering options at
restore time.

Another kind of file attributes is XFS-style project id as of
xfs_quota(8). These ids are numbers used for marking files for the
application of disk quotas. Also available in ext. See man 5 ext4,
properties "project" and "quota".
These numbers are individual to the quota setup of an individual
filesystem. So the backup program has at restore time to offer ways of
mapping them to appropriate numbers of the target filesystem.

(I spent some time with teaching xorriso-1.5.7 how to record and
restore chattr flags and project ids when xorriso setting -for_backup
is active. Now i need to find the energy to test ISO 9660 17-byte
timestamps with kernel 6.16+ and then to release xorriso-1.5.8.
The reward for me will be that users discover embarrassing bugs which
slipped through all my testing ...)


Have a nice day :)

Thomas