ref: c3101e4da486b2bf2e8b911bcb62dd8ec31c41bf
dir: /man/4/dbfs/
.TH DBFS 4
.SH NAME
dbfs, rawdbfs \- simple database file system
.SH SYNOPSIS
.B dbfs
[
.B -abcer
]
.I file mountpoint
.br
.B rawdbfs
[
.B -abcelx
]
[
.B -u
.I cmd
]
.I file mountpoint
.PP
.IB mountpoint /new
.br
.IB mountpoint /0
.br
.IB mountpoint /1
.br
.IB mountpoint /\fR...\fP
.SH DESCRIPTION
.B Dbfs
and
.B rawdbfs
both expose a simple, record-based filesystem stored
in
.IR file .
The
.BR -a ,
.B -b
and
.B -c
options have the same meaning as the options accepted by
.IR bind (1).
If the
.B -e
option is given, then
.I file
will be created if it does not already exist.
The filesystem provided by both
.B dbfs
and
.B rawdbfs
is substantially the same:
when started,
.I mountpoint
is populated with numbered files, one for each record found in
.IR file .
A read of one of these files yields the data in the record;
a write stores data in the record. A new record can be created
by opening the
.B new
file; writes to this file write to the new record. The name
of the new file can be discovered by using
.I fstat
in
.IR sys-stat (2).
Records can be removed by removing the appropriate
record file.
.PP
.B Rawdbfs
has additional features for database use:
.BR -x
causes two additional files ("index" and "stats") to appear
in the mounted directory,
.BR -l
specifies that a record can only be opened for writing
by one process at once, and
.BR -u
.I cmd
specifies a
.IR sh (1)
command to be run whenever the contents of the database
change.
The index file is provided as a convenience for database
client applications. Once initialised (by writing an integer value to it),
subsequent reads return an ever-increasing integer value.
Reading the stats file returns counts of database
read, write, create and delete operations as a string of four
integers.
.PP
.B Dbfs
and
.B rawdbfs
differ in the way that they store their data.
.B Dbfs
stores its records in
.I file
in a simple text format: the end of a record is indicated
by an empty line. The file is completely rewritten every time
a record is written. Storing records containing blank lines
will lead to confusion when the database is re-read.
.B Rawdbfs
can store arbitrary data, but the format of the data
storage is known only to itself. It does not rewrite
the whole file on every record change, so can be more
suitable for flash-based storage, where it is important
to minimise the number of writes.
.PP
Note that the record numbers always get their initial
numbering from the order of the records in
.IR file .
You cannot assume that the filename given to a record
will remain the same between runs of
.BR dbfs .
.SH SOURCE
.B /appl/cmd/dbfs.b
.br
.B /appl/cmd/rawdbfs.b
.SH SEE ALSO
.IR memfs (4),
.IR ramfile (4),
.IR calendar (1)
.SH BUGS
Write offsets are ignored, so the maximum amount of data
that can be stored in a record is ATOMICIO bytes (i.e. 8K).
.PP
There is no way to compact a file maintained by
.BR dbfs .