.TH BACKUP 5
.CT 1 sa_mortals 
.SH NAME
backup \- incremental backup files
.SH DESCRIPTION
The backup system consists of a number of client machines, and a
`backup machine',
which has a database and a collection of
backup copies of files.
On clients files concerned with backup live in a directory, normally
.FR /usr/lib/backup ,
defined in the shell script
.FR /usr/bin/backup .
The file
.FR /usr/lib/backup/conf ,
needed on both client and backup machine,
has up to three lines, namely the backup machine name,
the default backup device, and the directory, hereafter called
.BR $FM ,
where the rest of the backup software
lives on the backup machine.
Client systems normally have just the first line.
The rest of this description applies to the backup machine.
.PP
The databases live in
.BR $FM/db 
and are maintained in
.IR cbt (3)
form.
The main database, called
.FR filemap ,
stores two mappings.
The first maps filename-time pairs to backup copy names, thus:
.IP
.EX
/n/bowell/usr/jim/goo//519487622   v/v22/17
.EE
.LP
The number after
.CW //
is the inode change date,
expressed in seconds since the epoch; see
.IR stat (2).
If the backup copy is still on magnetic disk, it will be called
.BR $FM/v/v22/17 ;
otherwise it will be
.B v22/17
on some optical disk.
(The mapping of backup copy name to optical disk name is kept in 
.BR $FM/adm/volidmap .)
The second mapping maps filenames to the time of their most recently backed-up version:
.IP
.L
/n/bowell/usr/jim/goo   520514116
.PP
The second database,
.BR dir ,
maps directoryname-time pairs to the contents of that directory.
This allows quick recovery of file trees.
.PP
The third database,
.BR fs ,
maps filename-time pairs to (essentially) inodes.
This allows efficient implementation of 
.BR "backup mount" ;
see
.IR backup (1).
.PP
The program
.B $FM/bin/dbupdate
manages these databases.
The
.I dir
and
.I fs 
databases are optional; they will be updated only if they already exist.
The program
.F "$FM/bin/sweep"
also assigns the backup copy names into a flat
directory structure.
A new directory is used when the total size of the files in the current directory
would exceed 20000K
bytes, rounding each file size up to a multiple of 4K.
.PP
The backup copy of a file consists of a header
that gives the original inode, pathname and owner (as a string),
followed by the contents of the file.
Directories are stored as a sequence of entry names.
.PP
To prevent multiple writers into a database,
a lockfile
.FI $FM/locks/ database
is used.
The content of this file
is the process id of the process accessing the database.
Locks are removed by
.F $FM/bin/rmlocks
executed by
.IR rc (8)
when the system boots.
.PP
The backup system supports multiple
.I filemap
databases (this allows the current database to be kept small).
The list of database names is kept in
.FR $FM/db/filemaplist ,
one per line in order of increasing priority.
The last name is assumed to be the active database; all the
others are read-only.
.PP
Programs such as
.I sweep
and
.I dbupdate
leave droppings in the log file
.FR $FM/log .
.PP
Statistics of the numbers of files and bytes saved for users 
of a given system are kept in
.FR $FM/stat.dir/\fIsystem .
Each file consists of a sequence of records with a machine-independent structure;
generally, one record per user per day.
The records are maintained by
.FR "$FM/bin/procstats" ,
which processes the file
.F $FM/stat.log
that is maintained by
.IR dbupdate .
.PP
To allow quick searching for filenames with full regular expressions,
a simple sorted list of all saved filenames is often kept (normally) in
.FR $FM/filenames .
.PP
The device (and system) used for recovering files can be specified in many ways.
In order of decreasing priority:
a
.B -f
option in
.I backup recover
or
.I backup fetch
(see
.IR backup (1)),
a default device on the client system (in line 2 of
.FR /usr/lib/backup/conf ),
the default device on the backup system.
.SH FILES
.F /usr/lib/backup/*
.br
.F /usr/lib/backup/conf
.br
.F /usr/backup/db
.br
.F /usr/backup/locks
.br
.F /usr/backup/log
.br
.F /usr/backup/filenames
.SH "SEE ALSO"
.IR backup (1),
.IR worm (8),
.IR backup (8),
.IR cbt (1),
.IR stat (2)
.br
A. Hume,
`The File Motel: an Owner's Manual',
this manual, Volume 2