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A snapshot file (or directory file) is created during
incremental backups (see section Using
tar to Perform Incremental Dumps). It
contains the status of the file system at the time of the dump and is
used to determine which files were modified since the last backup.
tar version 1.34 supports three snapshot file
formats. The first format, called format 0, is the one used by
tar versions up to and including 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. It was used by alpha release
version 1.15.90. For alpha version 1.15.91 and stable releases
version 1.16 up through 1.34, the format 2 is used.
tar is able to read all three formats, but will create
snapshots only in format 2.
This appendix describes all three formats in detail.
[nfs]dev inode name
A single plus character (‘+’), if this directory is located on an NFS-mounted partition, otherwise empty.
(That is, for non-NFS directories, the first character on the description line contains the start of the dev field.)
Device number of the directory;
I-node number of the directory;
Name of the directory. Any special characters (white-space, backslashes, etc.) are quoted.
where tar-version is the version number of GNU
implementation that created this snapshot, and
incr-format-version is the version number of the snapshot format
(in this case ‘1’).
Next 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 epoch.
Lines that follow contain directory metadata, one line per directory. Each line is formatted as follows:
[nfs]mtime-sec mtime-nsec dev inode name
where mtime-sec and mtime-nsec represent last modification time of this directory with nanosecond precision; nfs, dev, inode and name have the same meaning as with ‘format 0’.
This line is followed by newline. Rest of file consists of records, separated by null (ASCII 0) characters. Thus, in contrast to the previous formats, format 2 snapshot is a binary file.
First two records are decimal integers, 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 epoch. These are followed by arbitrary number of directory records.
Each directory record contains a set of metadata describing a particular directory. Parts of a directory record are delimited with ASCII 0 characters. The following table describes each part. The Number type in this table stands for a decimal integer in ASCII notation. (Negative values are preceded with a "-" character, while positive values have no leading punctuation.)
|nfs||Character||‘1’ if the directory is located on an NFS-mounted partition, or ‘0’ otherwise;|
|timestamp_sec||Number||Modification time, seconds;|
|timestamp_nsec||Number||Modification time, nanoseconds;|
|name||String||Directory name; in contrast to the previous versions it is not quoted;|
|contents||Dumpdir||Contents of the directory; See section Dumpdir, for a description of its format.|
Dumpdirs stored in snapshot files contain only records of types ‘Y’, ‘N’ and ‘D’.
The specific range of values allowed in each of the Number fields
depends on the underlying C datatypes as determined when
is compiled. To see the specific ranges allowed for a particular
tar binary, you can use the
$ tar --show-snapshot-field-ranges This tar's snapshot file field ranges are (field name => [ min, max ]): nfs => [ 0, 1 ], timestamp_sec => [ -9223372036854775808, 9223372036854775807 ], timestamp_nsec => [ 0, 999999999 ], dev => [ 0, 18446744073709551615 ], ino => [ 0, 18446744073709551615 ],
(This example is from a GNU/Linux x86_64 system.)
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