Description of problem:
According to tar man page:

Device blocking:
       -b, --blocking-factor=BLOCKS
              BLOCKS x 512 bytes per record

and tar will read and write entire records.

According to tarfile documentation,

tarfile.open(name=None, mode=’r’, fileobj=None, bufsize=10240, **kwargs)
...
bufsize specifies the blocksize and defaults to 20 * 512 bytes

but if I try creating an archive specifying a different blocking factor, tarfile simply ignores and keeps 10240.

The issue can be easily reproduced with this snippet:

    import os
    import tarfile
    import tempfile
    
    EXPECTED_SIZE = 20480
    tempdir = tempfile.gettempdir()
    fd, _tmp_tar = tempfile.mkstemp(
        suffix='.tar',
        dir=tempdir,
    )
    os.close(fd)
    tar = tarfile.open(name=_tmp_tar, mode='w', bufsize=EXPECTED_SIZE)
    tar.close()
    statinfo = os.stat(_tmp_tar)
    if statinfo.st_size != EXPECTED_SIZE:
        raise RuntimeError((
            'Archive size doens\'t match expected size: '
            'actual {a} - expected {e}'
        ).format(
            a=statinfo.st_size,
            e=EXPECTED_SIZE,
        ))


Version-Release number of selected component (if applicable):


How reproducible:
python-libs-2.7.5-58.el7.x86_64

Steps to Reproduce:
1. try creating an archive with a custom blocking factor
2.
3.

Actual results:
tarfile.open ignores and keeps 10240 (the default value)

Expected results:
tarfile.open honors the custom blocking factor

Additional info:

    tar --blocking-factor=40 -cf test.tar -T /dev/null

works as expected creating an empty tar archive of 20480 (40x512) bytes.