.TH FMTINDEX 8
.SH NAME
fmtindex \- format a Venti index
.SH SYNOPSIS
.B venti/fmtindex
[
.B -a
]
.I venti.config
.SH DESCRIPTION
.I Fmtindex
takes the
.IR venti.conf (6)
file
.I venti.config
and initializes the index sections to form a usable index structure.
The arena files and index sections must have previously been formatted
using 
.IR fmtarenas (8)
and 
.IR fmtisect (8)
respectively.
.PP
The function of a Venti index is to map a Sha1 fingerprint to a location
in the data section of one of the arenas.  The index is composed of
blocks, each of which contains the mapping for a fixed range of possible
fingerprint values.
.I Fmtindex
determines the mapping between Sha1 values and the blocks
of the collection of index sections.  Once this mapping has been determined,
it cannot be changed without rebuilding the index. 
The basic assumption in the current implementation is that the index
structure is sufficiently empty that individual blocks of the index will rarely
overflow.  The total size of the index should be about 2% to 10% of
the total size of the arenas, but the exact depends both the index block size
and the compressed size of block stored to Venti.
.PP
.I Fmtindex
also computes a mapping between a linear address space and
the data section of the collection of arenas.  The
.B -a
option can be used to add additional arenas to an index.
To use this feature,
add the new arenas to
.I venti.config
after the existing arenas and then run
.I fmtindex
.BR -a .
.PP
A copy of the above mappings is stored in the header for each of the index sections.
These copies enable
.IR buildindex (8)
to restore a single index section without rebuilding the entire index.
.SH SOURCE
.B /sys/src/cmd/venti
.SH "SEE ALSO"
.IR venti (8),
.IR fmtarenas (8),
.IR fmtisect (8),
.IR venti.conf (6)