195 lines
5.6 KiB
Plaintext
195 lines
5.6 KiB
Plaintext
.SH
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Block Devices
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.PP
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The block device I/O system is like a
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protocol stack of filters.
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There are a set of pseudo-devices that call
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recursively to other pseudo-devices and real devices.
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The protocol stack is compiled from a configuration
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string that specifies the order of pseudo-devices and devices.
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Each pseudo-device and device has a set of entry points
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that corresponds to the operations that the file system
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requires of a device.
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The most notable operations are
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.CW read ,
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.CW write ,
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and
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.CW size .
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.PP
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The device stack can best be described by
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describing the syntax of the configuration string
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that specifies the stack.
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Configuration strings are used
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during the setup of the file system.
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For a description see
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.I fsconfig (8).
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In the following recursive definition,
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.I D
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represents a
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string that specifies a block device.
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.IP "\fID\fP = (\fIDD\fP...)"
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.br
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This is a set of devices that
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are concatenated to form a single device.
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The size of the catenated device is the
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sum of the sizes of each sub-device.
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.IP "\fID\fP = [\fIDD\fP...]"
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.br
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This is the interleaving of the
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individual devices.
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If there are N devices in the list,
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then the pseudo-device is the N-way block
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interleaving of the sub-devices.
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The size of the interleaved device is
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N times the size of the smallest sub-device.
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.IP "\fID\fP = {\fIDD\fP...}"
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.br
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This is a set of devices that
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constitute a `mirror' of the first sub-device, and form a single device.
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A write to the device is performed,
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at the same block address,
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on the sub-devices, in right-to-left order.
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A read from the device is performed on each sub-device,
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in left-to-right order, until a read succeeds without error,
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or the set is exhausted.
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One can think of this as a poor man's RAID 1.
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The size of the device is the size of the smallest sub-device.
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.IP "\fID\fP = \f(CWp\fP\fIDN1.N2\fP"
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.br
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This is a partition of a sub-device.
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The sub-device is partitioned into 100 equal pieces.
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If the size of the sub-device is not divisible by 100,
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then there will be some slop thrown away at the top.
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The pseudo-device starts at the N1-th piece and
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continues for N2 pieces. Thus
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.CW p\fID\fP67.33
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will be the
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last third of the device
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.I D .
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.IP "\fID\fP = \f(CWf\fP\fID\fP"
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.br
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This is a fake write-once-read-many device simulated by a
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second read-write device.
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This second device is partitioned
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into a set of block flags and a set of blocks.
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The flags are used to generate errors if a
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block is ever written twice or read without being written first.
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.IP "\fID\fP = \f(CWx\fP\fID\fP"
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.br
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This is a byte-swapped version of the file system on D.
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Since the file server currently writes integers in metadata to disk
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in native byte order, moving a file system to a machine of the other
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major byte order (e.g., MIPS to Pentium)
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requires the use of
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.CW x .
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It knows the sizes of the various integer fields in the file system metadata.
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Ideally, the file server would follow the Plan 9 religion and write a consistent
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byte order on disk, regardless of processor.
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In the mean time, it should be possible to automatically determine the need
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for byte-swapping by examining data in the super-block of each file system,
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though this has not been implemented yet.
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.IP "\fID\fP = \f(CWc\fP\fIDD\fP"
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.br
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This is the cache/WORM device made up of a cache (read-write)
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device and a WORM (write-once-read-many) device.
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More on this later.
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.IP "\fID\fP = \f(CWo\fP"
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.br
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This is the dump file system that is the
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two-level hierarchy of all dumps ever taken on a cache/WORM.
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The read-only root of the cache/WORM file system
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(on the dump taken Feb 18, 1995) can
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be referenced as
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.CW /1995/0218
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in this pseudo device.
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The second dump taken that day will be
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.CW /1995/02181 .
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.IP "\fID\fP = \f(CWw\fP\fIN1.N2.N3\fP"
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.br
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This is a SCSI disk on controller N1, target N2 and logical unit number N3.
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.IP "\fID\fP = \f(CWh\fP\fIN1.N2.0\fP"
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.br
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This is an (E)IDE or *ATA disk on controller N1, target N2
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(target 0 is the IDE master, 1 the slave device).
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These disks are currently run via programmed I/O, not DMA,
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so they tend to be slower to access than SCSI disks.
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.IP "\fID\fP = \f(CWr\fP\fIN1\fP"
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.br
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This is the same as
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.CW w ,
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but refers to a side of a WORM disc.
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See the
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.I j
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device.
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.IP "\fID\fP = \f(CWl\fP\fIN1\fP"
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.br
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This is the same as
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.CW r ,
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but one block from the SCSI disk is removed for labeling.
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.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"
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.br
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.I D\d\s-2\&1\s+2\u
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is the juke box SCSI interface.
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The
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.I D\d\s-2\&2\s+2\u 's
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are the SCSI drives in the juke box
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and the
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.I D\d\s-2\&3\s+2\u 's
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are the demountable platters in the juke box.
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.I D\d\s-2\&1\s+2\u
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and
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.I D\d\s-2\&2\s+2\u
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must be
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.CW w .
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.I D\d\s-2\&3\s+2\u
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must be pseudo devices of
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.CW w ,
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.CW r ,
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or
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.CW l
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devices.
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.PP
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For
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.CW w ,
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.CW h ,
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.CW l ,
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and
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.CW r
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devices any of the configuration numbers
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can be replaced by an iterator of the form
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.CW <\fIN1-N2\fP> .
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N1 can be greater than N2, indicating a descending sequence.
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Thus
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.Ex
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[w0.<2-6>]
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.Ee
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is the interleaved SCSI disks on SCSI targets
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2 through 6 of SCSI controller 0.
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The main file system on
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Emelie
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is defined by the configuration string
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.Ex
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c[w1.<0-5>.0]j(w6w5w4w3w2)(l<0-236>l<238-474>)
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.Ee
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This is a cache/WORM driver.
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The cache is three interleaved disks on SCSI controller 1
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targets 0, 1, 2, 3, 4, and 5.
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The WORM half of the cache/WORM
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is 474 jukebox disks.
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Another file server,
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.I choline ,
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has a main file system defined by
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.Ex
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c[w<1-3>]j(w1.<6-0>.0)(l<0-124>l<128-252>)
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.Ee
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The order of
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.CW w1.<6-0>.0
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matters here, since the optical jukebox's WORM drives's
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SCSI target ids,
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as delivered,
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run in descending order relative to the numbers of the drives
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in SCSI commands
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(e.g., the jukebox controller is SCSI target 6,
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drive #1 is SCSI target 5,
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and drive #6 is SCSI target 0).
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