[Bug-tar] GNU tar 1.15.1, format gnu, multivolume failure

[John Gatewood Ham]

GNU tar maintainers,

I have to backup my hard drive.  I normally use GNU tar.  It broke
on me when I tried to backup our server before an upgrade.

I have version 1.15.1 and normally it works, but I have learned
much to my horror while backing up my system that while
you claim to handle unlimited length file names but you can only
store 128 byte file names on a border.  Rather than handle it
gracefully you crash with an error.  Since I had already made it
through 7 CDs I am worried that when I rename that last file and
do the backup again I'll hit this bug many times (I have to backup
24GB, so many more chances for crashes).

> From the info file:
"gnu
       Format used by GNU `tar' versions up to 1.13.25. This format
       derived from an early POSIX standard, adding some improvements
       such as sparse file handling and incremental archives.
       Unfortunately these features were implemented in a way
       incompatible with other archive formats.

       Archives in `gnu' format are able to hold pathnames of unlimited
       length."

May I suggest instead: "Archives in `gnu' format are able to hold
pathnames of unlimited length when you do not use the multi-volume
option."

First I tried --format=posix since all your documentation says use
it.  That does not do multivolume backups.  I found that documented
in the TODO file, but not in the main info file.  The error message
was not helpful in this case.  On the chart where you compare
features you should have this instead I think:

"Format  UID            File Size      Path Name      Devn      Multiple
                                                                  Volumes
   gnu     1.8e19         Unlimited      Unlimited      63        Yes **
   oldgnu  1.8e19         Unlimited      Unlimited      63        ?
   v7      2097151        8GB            99             n/a       No
   ustar   2097151        8GB            256            21        ?
   posix   Unlimited      Unlimited      Unlimited      Unlimited No

** WARNING: the last file name on a volume in a multi-volume archive
              member must be strictly less than 129 bytes long."

If the bug is documented, at least people know they are playing russian 
roulette when they attempt a multi-volume backup.  The question marks
are for formats I do not know about.

I could make a single-volumne a.tar and then split that and back it up
with multi-volume tar since it has a short name, but I would
need 24+ GB of free space to do it, and I don't have that much free
space.  I would need the same amount of free space on restore too.

Thank you for producing tar and continuing development.  Perhaps you
could create a 1.15.2 version that can deal with this soon?  I
hope so.  The hard drives have always outstripped the size of backup
media and I think that trend will continue for the forseeable future,
so this problem will remain relavant.

The French program 'dar' does not have this problem and let me get
past this, but I hope in the future I have the option of using
GNU tar for multi-volume backups.  Still, the immediate need is
to document this someplace obvious so people can do something like:

    find /whatever -print | wc -L

and if the number is larger than 128 they know not to attempt
a multi-volume backup.

Sincerely,

John Ham