|Subject:||Feature request: make --multivolume and --update seeking between headers|
|Date:||Sun, 24 Oct 2021 17:50:56 +0200|
|User-agent:||Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0 Thunderbird/78.13.0|
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there any technical reasons for that, other than outdated silent
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?
-n, --seekAs 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.
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-
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