.TH FS 3
.SH NAME
fs \- file system devices
.SH SYNOPSIS
.nf
.B bind -b #k /dev
.sp 0.3v
.B /dev/fs
.B /dev/fs/ctl
.B /dev/fs/...
.B /dev/\fInew\fP
.fi
.SH DESCRIPTION
The
.I fs
driver builds complex disk files out of simpler disk files.
Inspired by the Plan 9 file server kernel's configuration strings,
it provides device mirroring, partitioning, interleaving, and catenation
for disk-based services like
.IR fossil (4)
or
.IR venti (8).
.PP
The device is intended to be bound at
.B /dev
and initially contains a directory named
.BR fs ,
which in turn contains a
.B ctl
file and one file per configured device.
.PP
Most control messages introduce a new device, here named
.IR new .
The
.I file
arguments are interpreted in the name space of the writing process.
.PP
The device name
.I new
may be a single filename component (containing no slashes);
in this case, the device is created under
.BR #k/fs .
If
.I new
instead has the format
.IB dir / file,
the device is made available at
.BI #k/ dir / file.
The directory
.I dir
goes away when the last device on it is removed with the
.B del
control message,
but
.B #k/fs
will never be removed.
.TF "del \fIold
.PD
.TP
.BI cat " new files" \fR...
The device
.I new
corresponds to the catenation of
.IR files .
.TP
.BI inter " new files" \fR...
The device
.I new
corresponds to the block interleaving of
.IR files ;
an 8192-byte block size is assumed.
.TP
.BI mirror " new files" \fR...
The device
.I new
corresponds to a RAID-1-like mirroring of
.IR files .
Writes to
.BI new
are handled by sequentially writing the same data to the
.I files
from right to left (the reverse of
the order in the control message).
A failed write causes an eventual error return
but does not prevent the rest of the writes
to the other devices of the mirror set.
Reads from
.BI new
are handled by sequentially reading from the
.I files
from left to right until one succeeds.
The length of the mirror device is the minimum of the lengths of the
.IR files .
.TP
.BI part " new file offset length"
.PD 0
.TP
.BI part " new offset end
.PD
In the first form,
the device
.I new
corresponds to the
.I length
units starting at
.I offset
in
.IR file .
If
.IR offset + length
reaches past the end of
.IR file ,
.I length
is silently reduced to fit.
Units are bytes.
In the second form,
a previous
.B disk
request must have defined the source
.I file
for further requests and the end of the device
is determined by the
.I end
offset in the source file, and not by the device
length. Units are as defined in the previous
.B disk
request. This form is accepted for compatibility with
.IR fdisk
(in
.IR prep (8))
and
.IR sd (3)
devices.
.TP
.BI del " old
Removes the device named
.IR old .
The device will still be seen while in use.
Further I/O attempts will fail with an error indication stating that
the device is gone.
When
.I old
is
.IB dir /*\fR,
all devices under
.I dir
are removed.
.TP
.BI disk " dir [ n file ]
makes
.I dir
implicit in new device names (i.e., it makes
.I new
mean
.IB dir / new
by default).
Optional argument
.I n
specifies the default unit (sector) size in bytes and the default source
.I file
for further partition devices.
Default values are restored when the control file is closed.
.TP
.B clear
Discard all
.I fs
device definitions.
.PD
.LP
If the variable
.B fsconfig
is set in
.IR plan9.ini (8),
.I fs
will read its configuration from the file
.B $fsconfig
on the first attach.
This is useful when the machine boots from a local file server that uses
.IR fs .
.SH EXAMPLES
Use a previously partitioned disk,
.BR /dev/sdC0 ,
making
partition files available under
.BR /dev/sdC0parts :
.IP
.EX
{
	echo disk sdC0parts 512 /dev/sdC0/data
	disk/fdisk -p /dev/sdC0/data
	# now create plan 9 partitions
	echo disk sdC0parts 512 /dev/sdC0parts/plan9
	disk/prep -p /dev/sdC0parts/plan9
} > /dev/fs/ctl
.EE
.LP
Mirror the two disks
.B /dev/sdC0/data
and
.B /dev/sdD0/data
as
.BR /dev/fs/m0 ;
similarly, mirror
.B /dev/sdC1/data
and
.B /dev/sdD1/data
as
.BR /dev/fs/m1 :
.IP
.EX
echo mirror m0 /dev/sdC0/data /dev/sdD0/data >/dev/fs/ctl
echo mirror m1 /dev/sdC1/data /dev/sdD1/data >/dev/fs/ctl
.EE
.LP
Interleave the two mirrored disks to create
.BR /dev/fs/data :
.IP
.EX
echo inter data /dev/fs/m0 /dev/fs/m1 >/dev/fs/ctl
.EE
.LP
Run
.IR kfs (4)
on the interleaved device:
.IP
.EX
disk/kfs -f /dev/fs/data
.EE
.LP
Save the configuration:
.IP
.EX
cp /dev/fs/ctl /dev/fd0disk
.EE
.LP
To load the configuration automatically at boot time,
add this to
.IR plan9.ini :
.IP
.EX
fsconfig=/dev/fd0disk
.EE
.SH "SEE ALSO"
.I read
in
.IR cat (1),
.IR dd (1),
.IR sd (3),
.IR fossil (4),
.IR fs (8),
.IR plan9.ini (8),
.IR prep (8),
.IR venti (8)
.SH SOURCE
.B /sys/src/9/port/devfs.c
.SH BUGS
Mirrors are RAID-like but not RAID.
There is no fancy recovery mechanism and
no automatic initial copying from a master drive to its mirror drives.
.PP
Each
.I write
system call on
.B ctl
may transmit at most one command.