tar checkpoints not matching input

Question

Consider the following command

tar acf file.tar.gz file-5.16 --checkpoint=1 --blocking-factor=50 \
  --checkpoint-action='ttyout=%u\r'

Here is the size of the input folder

$ du -bs file-5.16
2520025 file-5.16

Now with this command a checkpoint is passed every record, and a record in this case is
50 * 512 bytes = 25,600 bytes. It follows that 2520025 / 25600 ≈ 98 checkpoints. So why am I getting

109

As the output?

Answer 1

To start, you cannot just take the total bytes in the folder and divide it. You must work with the individual files. For each file you need to

ceil(bytes / 512) + 1 = blocks

The plus one is because each file has an extra block

at the end of the file there's a block containing all zeros

§ 9.4 Blocking

After that you add the blocks for all files and divide by the blocking factor, in this case 50

ceil(blocks / 50) = records

Some demo code

$ find file-5.16 -type f |
    xargs du -B512 --apparent-size |
    perl -MPOSIX -ane '$bk += $F[0]+1; END {print ceil $bk/50}'
109