make `tahoe backup` useable as a replacement for rsync

[chrysn]

there is a number of probems due to which tahoe backup can not replace rsync style backups yet. the core of the problem is that tahoe-lafs can not keep all the information that is stored in a posix style file system. the issues i see are:

• ctime/mtime is not saved
• symlinks can not be saved (compare ticket #641, which has been around for two years)
• other special files can not be saved (devices etc)
• user, group and permissions are not saved
• acls are not saved

i am aware that tahoe has its own ways of dealing with permissions, that it has its own time stamps, and that directories work in a way that every directory entry is kind of a link anyway, but that's not the point -- it's about being able to restore a disk's contents from a backup.

from my point of view, symlinks, times and user/group/permissions are the most important of these; device files are nowadays created on a ramdisk on the fly anyway, and acl users know the problem well enough to have their workarouds (afair this is an issue with most backup systems).

implementation-wise, i guess that most if not all of this can be stored in the directory as additional information.

if it is possible in trac, i suggest all related bugs to be marked as "blocking" this bug.

is this something that is realistic to achieve for tahoe-lafs?