fs – file system devices


bind -b #k /dev

unhandled troff command .sp

/dev/fs /dev/fs/ctl /dev/fs/... /dev/new


The 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 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 (containing 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 interleaving of files; an 8192-byte block size is assumed.  

mirror new files ...

The device new corresponds to a RAID-1-like mirroring of files. Writes to new are handled by sequentially writing the same data to the 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 new are handled by sequentially reading from the files from left to right until one succeeds. The length of the mirror device is the minimum 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 offset 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 argument n specifies the default unit (sector) size in bytes and the default source file for further partition devices. Default values are restored when the control file is closed.  

crypt new file key

The device new corresponds to a AES-encrypted partition file encrypted with key (see cryptsetup(8)).  


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.


Use a previously partitioned disk, /dev/sdC0, making partition 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 /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 hjfs(4) on the interleaved device:


hjfs -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:




read in cat(1), dd(1), sd(3), fs(8), plan9.ini(8), prep(8), venti(8)




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 command.