Appendix D Format of the Incremental Snapshot Files
A snapshot file (or directory file) is created during
incremental backups (see Incremental Dumps). It
contains the status of the filesystem at the time of the dump and is
used to determine which files were modified since the last backup.
GNU tar version 1.15.90 supports two snapshot file
formats. The first format, called format 0, is the one used by
GNU tar versions up to 1.15.1. The second format, called format
1 is an extended version of this format, that contains more metadata
and allows for further extensions.
‘Format 0’ snapshot file begins with a line containing a
decimal number that represents the UNIX timestamp of the beginning of
the last archivation. This line is followed by directory metadata
descriptions, one per line. Each description has the following format:
where optional nfs is a single plus character (‘+’) if this
directory is located on an NFS-mounted partition, dev and
inode are the device and inode numbers of the directory, and
name is the directory name.
‘Format 1’ snapshot file begins with a line specifying the
format of the file. This line has the following structure:
where tar-version is the version of GNU tar implementation
that created this snapshot, and incr-format-version is the
version number of the snapshot format (in this case ‘1’).
The following line contains two decimal numbers, representing the
time of the last backup. First number is the number of seconds, the
second one is the number of nanoseconds, since the beginning of the
Following lines contain directory metadate, one line per
directory. The line format is:
where mtime-sec and mtime-nsec represent the last
modification time of this directory with nanosecond precision;
nfs, dev, inode and name have the same meaning
as with ‘format 0’.
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