FS(3) FS(3)
NAME
fs - file system devices
SYNOPSIS
bind -b #k /dev
/dev/fs
/dev/fs/ctl
/dev/fs/...
/dev/new
DESCRIPTION
The fs driver builds complex disk files out of simpler disk
files. Inspired by the Plan 9 file server kernel's configu-
ration strings, it provides device mirroring, partitioning,
interleaving, and catenation for disk-based services like
fossil(4), kfs(4), or venti(8).
The device is intended to be bound at /dev and initially
contains a directory named fs, which in turn contains a ctl
file and one file per configured device.
Most control messages introduce a new device, here named
new. The file arguments are interpreted in the name space of
the writing process.
The device name new may be a single filename component (con-
taining no slashes); in this case, the device is created
under #k/fs. If new instead has the format dir/file, the
device is made available at #k/dir/file. The directory dir
goes away when the last device on it is removed with the del
control message, but #k/fs will never be removed.
cat new files...
The device new corresponds to the catenation of
files.
inter new files...
The device new corresponds to the block interleav-
ing of files; an 8192-byte block size is assumed.
mirror new files...
The device new corresponds to a RAID-1-like mirror-
ing of files. Writes to new are handled by sequen-
tially writing the same data to the files from
right to left (the reverse of the order in the con-
trol 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 new are handled by sequentially reading
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FS(3) FS(3)
from the files from left to right until one suc-
ceeds. The length of the mirror device is the min-
imum of the lengths of the files.
part new file offset length
part new offset end
In the first form, the device new corresponds to
the length units starting at offset in file. If
offset+length reaches past the end of file, length
is silently reduced to fit. Units are bytes. In
the second form, a previous disk request must have
defined the source file for further requests and
the end of the device is determined by the end off-
set in the source file, and not by the device
length. Units are as defined in the previous disk
request. This form is accepted for compatibility
with fdisk (in prep(8)) and sd(3) devices.
del old Removes the device named 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 old is dir/*, all devices
under dir are removed.
disk dir [ n file ]
makes dir implicit in new device names (i.e., it
makes new mean dir/new by default). Optional argu-
ment n specifies the default unit (sector) size in
bytes and the default source file for further par-
tition devices. Default values are restored when
the control file is closed.
clear Discard all fs device definitions.
If the variable fsconfig is set in plan9.ini(8), fs will
read its configuration from the file $fsconfig on the first
attach. This is useful when the machine boots from a local
file server that uses fs.
EXAMPLES
Use a previously partitioned disk, /dev/sdC0, making parti-
tion files available under /dev/sdC0parts:
{
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
Mirror the two disks /dev/sdC0/data and /dev/sdD0/data as
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FS(3) FS(3)
/dev/fs/m0; similarly, mirror /dev/sdC1/data and
/dev/sdD1/data as /dev/fs/m1:
echo mirror m0 /dev/sdC0/data /dev/sdD0/data >/dev/fs/ctl
echo mirror m1 /dev/sdC1/data /dev/sdD1/data >/dev/fs/ctl
Interleave the two mirrored disks to create /dev/fs/data:
echo inter data /dev/fs/m0 /dev/fs/m1 >/dev/fs/ctl
Run kfs(4) on the interleaved device:
disk/kfs -f /dev/fs/data
Save the configuration:
cp /dev/fs/ctl /dev/fd0disk
To load the configuration automatically at boot time, add
this to plan9.ini:
fsconfig=/dev/fd0disk
SEE ALSO
read in cat(1), dd(1), sd(3), fossil(4), kfs(4), fs(8),
plan9.ini(8), prep(8), venti(8)
SOURCE
/sys/src/9/port/devfs.c
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.
Each write system call on ctl may transmit at most one com-
mand.
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