harvey/sys/doc/fs/p4

.SH
Block Devices
.PP
The block device I/O system is like a
protocol stack of filters.
There are a set of pseudo-devices that call
recursively to other pseudo-devices and real devices.
The protocol stack is compiled from a configuration
string that specifies the order of pseudo-devices and devices.
Each pseudo-device and device has a set of entry points
that corresponds to the operations that the file system
requires of a device.
The most notable operations are
.CW read ,
.CW write ,
and
.CW size .
.PP
The device stack can best be described by
describing the syntax of the configuration string
that specifies the stack.
Configuration strings are used
during the setup of the file system.
For a description see
.I fsconfig (8).
In the following recursive definition,
.I D
represents a
string that specifies a block device.
.IP "\fID\fP = (\fIDD\fP...)"
.br
This is a set of devices that
are concatenated to form a single device.
The size of the catenated device is the
sum of the sizes of each sub-device.
.IP "\fID\fP = [\fIDD\fP...]"
.br
This is the interleaving of the
individual devices.
If there are N devices in the list,
then the pseudo-device is the N-way block
interleaving of the sub-devices.
The size of the interleaved device is
N times the size of the smallest sub-device.
.IP "\fID\fP = \f(CWp\fP\fIDN1.N2\fP"
.br
This is a partition of a sub-device.
The sub-device is partitioned into 100 equal pieces.
If the size of the sub-device is not divisible by 100,
then there will be some slop thrown away at the top.
The pseudo-device starts at the N1-th piece and
continues for N2 pieces. Thus
.CW p\fID\fP67.33
will be the
last third of the device
.I D .
.IP "\fID\fP = \f(CWf\fP\fID\fP"
.br
This is a fake write-once-read-many device simulated by a
second read-write device.
This second device is partitioned
into a set of block flags and a set of blocks.
The flags are used to generate errors if a
block is ever written twice or read without being written first.
.IP "\fID\fP = \f(CWc\fP\fIDD\fP"
.br
This is the cache/WORM device made up of a cache (read-write)
device and a WORM (write-once-read-many) device.
More on this later.
.IP "\fID\fP = \f(CWo\fP"
.br
This is the dump file system that is the
two-level hierarchy of all dumps ever taken on a cache/WORM.
The read-only root of the cache/WORM file system
(on the dump taken Feb 18, 1995) can
be referenced as
.CW /1995/0218
in this pseudo device.
The second dump taken that day will be
.CW /1995/02181 .
.IP "\fID\fP = \f(CWw\fP\fIN1.N2\fP"
.br
This is a SCSI disk on controller N1 and target N2.
.IP "\fID\fP = \f(CWl\fP\fIN1.N2\fP"
.br
This is the same as
.CW w ,
but one block from the SCSI disk is removed for labeling.
.IP "\fID\fP = \f(CWj(\fP\fID\d\s-2\&1\s+2\u\fID\d\s-2\&2\s+2\u\f(CW*)\fID\d\s-2\&3\s+2\u\f1"
.br
.I D\d\s-2\&1\s+2\u
is the juke box SCSI interface.
The
.I D\d\s-2\&2\s+2\u 's
are the SCSI drives in the juke box
and  the
.I D\d\s-2\&3\s+2\u 's
are the demountable platters in the juke box.
.I D\d\s-2\&1\s+2\u
and
.I D\d\s-2\&2\s+2\u
must be
.CW w .
.I D\d\s-2\&3\s+2\u
must be pseudo devices of
.CW w
or
.CW l
devices.
.PP
For both
.CW w
and
.CW r
devices any of the configuration numbers
can be replaced by an iterator of the form
.CW <\fIN1-N2\fP> .
Thus
.P1
    [w0.<2-6>]
.P2
is the interleaved SCSI disks on SCSI targets
2 through 6 of SCSI controller 0.
The main file system on
Emelie
is defined by the configuration string
.P1
    c[w1.<0-5>.0]j(w6w5w4w3w2)l(<0-236>l<238-474>)
.P2
This is a cache/WORM driver.
The cache is three interleaved disks on SCSI controller 1
targets 0, 1, 2, 3, 4, and 5.
The WORM half of the cache/WORM
is 474 jukebox disks.