From info tar:
The checkpoint facility is enabled using the following option:
--checkpoint[=N]: Schedule checkpoints before writing or reading each Nth record. The default value for N is 10.
So the default value for N is 10 records. But what is a record anyway?
In truth, the meaning of record above is not easily discerned. There are no hints or pointers in the checkpoint section of the info tar manual. Still if you go further on you'll eventually come to the section on blocks and blocking-factor.
The data in an archive is grouped into blocks, which are 512 bytes.
Blocks are read and written in whole number multiples called records.
The number of blocks in a record (i.e., the size of a record in units
of 512 bytes) is called the blocking factor. The
--blocking-factor=512-SIZE (-b 512-SIZE) option specifies the
blocking factor of an archive. The default blocking factor is
typically 20 (i.e., 10240 bytes), but can be specified at installation.
To find out the blocking factor of an existing archive, use tar --list
--file=ARCHIVE-NAME. This may not work on some devices.
So each checkpoint record is so-many blocks. This is definable via GNU tar's -b or --blocking-factor=[recordsize] option. If you do:
tar --show-defaults
You should get output like:
--format=gnu -f- -b20 --quoting-style=escape --rmt-command=/usr/lib/tar/rmt
Which would indicate that one record is 20 blocks.
You can also directly specify the record size in terms of bytes like:
--record-size=SIZE[SUF]
Instructs tar to use SIZE bytes per record when accessing the archive. The argument can be suffixed with a size suffix, e.g. --record-size=10K for 10 Kilobytes.
