Feature request: make --multivolume and --update seeking between headers

[Johannes Nieß]

Hi,

While working on a backup script that --updates the tar file (on a disk) with --multi-volume, I discovered that tar does not seek through the archive and speed is much lower than expected. Are there any technical reasons for that, other than outdated silent assumptions?

While trying to read the code and documentation, I stumbled upon this code in buffer.c

>   if (!multi_volume_option && !use_compress_program_option
>       && fstat (archive, &st) == 0)
>     seekable_archive = S_ISREG (st.st_mode);
>   else
>     seekable_archive = false;
>

That  multi_volume_option isn't documented to make the file non-seekable (see below). Is this just a silent and incorrect assumption that --multi-volume always implies non-seekable tapes?

    From the man page:

>        -n, --seek
>                       Assume  the  archive  is seekable.  Normally tar
>         determines automatically whether the archive
>                       can be seeked or not.  This option is intended for
>         use in cases when such recognition  fails.
>                       It  takes  effect  only if the archive is open for
>         reading (e.g. with --list or --extract op-
>                       tions).
>         
>       
      As far as I understand it, --update is mostly a combination of
      --compare (which is a seeking read operation) and --append in case
      file size and date differ. According to my tests, the --compare
      part of --update does not seek between headers (even without
      --multivolume and for an uncompressed .tar file).  Can we please
      get a huge performance boost in --update by making it jump from
      header to header (=seek) in the compare phase? The streamed file
      contents seem not to be needed for anything and slow down the
      process.

Best regards,

Johannes Nieß