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+.TH VAC 1
+.SH NAME
+vac \- create a vac archive on Venti
+.SH SYNOPSIS
+.B vac
+[
+.B -mqsv
+] [
+.B -b
+.I blocksize
+] [
+.B -d
+.I oldvacfile
+] [
+.B -e
+.I exclude
+] [
+.B -f
+.I vacfile
+] [
+.B -h
+.I host
+]
+.I file ...
+.SH DESCRIPTION
+.I Vac
+creates an archival copy of Plan 9 file trees on Venti. It can be used
+to build a simple backup system. One of the unusual properties of Venti is
+that duplicate blocks are detected and coalesced.  When
+.I vac
+is used on a file tree that shares data with an existing archive, the consumption of
+storage will be approximately equal to an incremental backup.
+This reduction in storage consumption occurs transparently to the user.
+.PP
+As an optimization, the 
+.B -d
+and
+.B -q
+options, described below, can be used to explicitly create an archive relative to an existing archive.
+These options do not change the resulting archive generated by
+.IR vac ,
+but simply reduce the number of write operations to Venti.
+.PP
+The output of
+.I vac
+is the hexadecimal representation of the Sha1 fingerprint of the root of the archive, in this format:
+.IP
+.EX
+vac:64daefaecc4df4b5cb48a368b361ef56012a4f46
+.EE
+.PP
+Option to 
+.I vac
+are:
+.TP
+.BI -b " blocksize
+Specifies the block size that data will be broken into.
+The units for the size can be specified by appending
+.L k
+to indicate kilobytes.
+The default is 8k.
+The size must be in the range
+of 512 bytes to 52k.
+.TP
+.BI -d " oldvacfile
+Reduce the number of blocks written to Venti by comparing the files to be stored with
+the contents of an existing
+.I vac
+file tree given by
+.IR oldvacfile .
+.TP
+.BI -e " exclude
+Do not include the file or directory specified by
+.IR exclude .
+This option may be repeated multiple times.
+.TP
+.BI -f " vacfile
+The results of 
+.I vac
+are place in
+.IR vacfile ,
+or the standard output if no file is given.
+.TP
+.BI -h " host
+The network address of the Venti server.
+The default is taken from the environment variable
+.BR venti .
+If this variable does not exist, then the default is the
+metaname 
+.BR $venti ,
+which can be configured via
+.IR ndb (6).
+.TP
+.B -m
+Expand and merge any
+.I vac
+archives that are found while reading the input files.  This option is
+useful for building an archive from a collection of existing archives.  Each archive is inserted
+into the new archive as if it had been unpacked in the directory in which it was found.  Multiple
+archives can be unpacked in a single directory and the contents will be merged.  To be detected, the
+archives must end in
+.LR .vac .
+Note, an archive is inserted by simply copying the root fingerprint and does not require
+the archive to be unpacked.
+.TP
+.B -q
+Increase the performance of the
+.B -d
+option by detecting unchanged files based on a match of the files name and other meta data,
+rather than examining the contents of the files.
+.TP
+.B -s
+Print out various statistics on standard error.
+.TP
+.B -v
+Produce more verbose output on standard error, including the name of the files added to the archive
+and the vac archives that are expanded and merged.
+.SH SOURCE
+.B /sys/src/cmd/vac
+.SH "SEE ALSO"
+.IR vacfs (4),
+.IR venti (8)