amdref.texinfo

早期freebsd实现

A @dfn{location-selection} is a list of possible volumes with which to
satisfy the request.  @dfn{location-selection}s are separated by the
@samp{||} operator.  The effect of this operator is to prevent use of
location-selections to its right if any of the location-selections on
its left were selected whether or not any of them were successfully
mounted (@pxref{Selectors}).@refill

The location-selection, and singleton @dfn{location-list},
@samp{type:=ufs;dev:=/dev/xd1g} would inform @i{Amd} to mount a UFS
filesystem from the block special device @file{/dev/xd1g}.

The @dfn{sel-or-opt} component is either the name of an option required
by a specific filesystem, or it is the name of a built-in, predefined
selector such as the architecture type.  The value may be quoted with
double quotes @samp{"}, for example
@samp{type:="ufs";dev:="/dev/xd1g"}.  These quotes are stripped when the
value is parsed and there is no way to get a double quote into a value
field.  Double quotes are used to get white space into a value field,
which is needed for the program filesystem (@pxref{Program Filesystem}).@refill

@menu
* Map Defaults::
* Variable Expansion::
* Selectors::
* Map Options::
@end menu

@node     Map Defaults, Variable Expansion, Location Format, Location Format
@comment  node-name,  next,  previous,  up
@subsection Map Defaults
@cindex Map defaults
@cindex How to set default map parameters
@cindex Setting default map parameters

A location beginning with a dash @samp{-} is used to specify default
values for subsequent locations.  Any previously specified defaults in
the location-list are discarded.  The default string can be empty in
which case no defaults apply.

The location @samp{-fs:=/mnt;opts:=ro} would set the local mount point
to @file{/mnt} and cause mounts to be read-only by default.  Defaults
specified this way are appended to, and so override, any global map
defaults given with @samp{/defaults}).
@c
@c A @samp{/defaults} value @dfn{gdef} and a location list
@c \begin{quote}
@c $@samp{-}@dfn{def}_a $\verb*+ +$ @dfn{loc}_{a_1} $\verb*+ +$ @dfn{loc}_{a_2} $\verb*+ +$ @samp{-}@dfn{def}_b $\verb*+ +$ @dfn{loc}_{b_1} \ldots$
@c \end{quote}
@c is equivalent to
@c \begin{quote}
@c $@samp{-}@dfn{gdef}@samp{;}@dfn{def}_a $\verb*+ +$ @dfn{loc}_{a_1} $\verb*+ +$ @dfn{loc}_{a_2} $\verb*+ +$ @samp{-}@dfn{gdef}@samp{;}@dfn{def}_b $\verb*+ +$ @dfn{loc}_{b_1} \ldots$
@c \end{quote}
@c which is equivalent to
@c \begin{quote}
@c $@dfn{gdef}@samp{;}@dfn{def}_a@samp{;}@dfn{loc}_{a_1} $\verb*+ +$@dfn{gdef}@samp{;}@dfn{def}_a@samp{;}@dfn{loc}_{a_2} $\verb*+ +$@dfn{gdef}@samp{;}@dfn{def}_b@samp{;}@dfn{loc}_{b_1} \ldots$
@c \end{quote}

@node     Variable Expansion, Selectors, Map Defaults, Location Format
@comment  node-name,  next,  previous,  up
@subsection Variable Expansion
@cindex Variable expansion
@cindex How variables are expanded
@cindex Pathname operators
@cindex Domain stripping
@cindex Domainname operators
@cindex Stripping the local domain name
@cindex Environment variables
@cindex How to access environment variables in maps

To allow generic location specifications @i{Amd} does variable expansion
on each location and also on some of the option strings.  Any option or
selector appearing in the form @code{$@dfn{var}} is replaced by the
current value of that option or selector.  For example, if the value of
@code{$@{key@}} was @samp{bin}, @code{$@{autodir@}} was @samp{/a} and
@code{$@{fs@}} was `@t{$@{autodir@}}@t{/local/}@t{$@{key@}}' then
after expansion @code{$@{fs@}} would have the value @samp{/a/local/bin}.
Any environment variable can be accessed in a similar way.@refill

Two pathname operators are available when expanding a variable.  If the
variable name begins with @samp{/} then only the last component of
then pathname is substituted.  For example, if @code{$@{path@}} was
@samp{/foo/bar} then @code{$@{/path@}} would be expanded to @samp{bar}.
Similarly, if the variable name ends with @samp{/} then all but the
last component of the pathname is substituted.  In the previous example,
@code{$@{path/@}} would be expanded to @samp{/foo}.@refill

Two domain name operators are also provided.  If the variable name
begins with @samp{.} then only the domain part of the name is
substituted.  For example, if @code{$@{rhost@}} was
@samp{swan.doc.ic.ac.uk} then @code{$@{.rhost@}} would be expanded to
@samp{doc.ic.ac.uk}.  Similarly, if the variable name ends with @samp{.}
then only the host component is substituted.  In the previous example,
@code{$@{rhost.@}} would be expanded to @samp{swan}.@refill

Variable expansion is a two phase process.  Before a location is parsed,
all references to selectors, @i{eg} @code{$@{path@}}, are expanded.  The
location is then parsed, selections are evaluated and option assignments
recorded.  If there were no selections or they all succeeded the
location is used and the values of the following options are expanded in
the order given: @var{sublink}, @var{rfs}, @var{fs}, @var{opts},
@var{remopts}, @var{mount} and @var{unmount}.

Note that expansion of option values is done after @dfn{all} assignments
have been completed and not in a purely left to right order as is done
by the shell.  This generally has the desired effect but care must be
taken if one of the options references another, in which case the
ordering can become significant.

There are two special cases concerning variable expansion:

@enumerate
@item
before a map is consulted, any selectors in the name received
from the kernel are expanded.  For example, if the request from the
kernel was for `@t{$@{arch@}}@t{.bin}' and the machine architecture
was @samp{vax}, the value given to @code{$@{key@}} would be
@samp{vax.bin}.@refill

@item
the value of @code{$@{rhost@}} is expanded and normalized before the
other options are expanded.  The normalization process strips any local
sub-domain components.  For example, if @code{$@{domain@}} was
@samp{Berkeley.EDU} and @code{$@{rhost@}} was initially
@samp{snow.Berkeley.EDU}, after the normalization it would simply be
@samp{snow}.  Hostname normalization is currently done in a
@emph{case-dependent} manner.@refill
@end enumerate

@node     Selectors, Map Options, Variable Expansion, Location Format
@comment  node-name,  next,  previous,  up
@subsection Selectors
@cindex Selectors

Selectors are used to control the use of a location.  It is possible to
share a mount map between many machines in such a way that filesystem
location, architecture and operating system differences are hidden from
the users.  A selector of the form @samp{arch==sun3;os==sos4} would only
apply on Sun-3s running SunOS 4.x.

Selectors are evaluated left to right.  If a selector fails then that
location is ignored.  Thus the selectors form a conjunction and the
locations form a disjunction.  If all the locations are ignored or
otherwise fail then @i{Amd} uses the @dfn{error} filesystem
(@pxref{Error Filesystem}).  This is equivalent to having a location
@samp{type:=error} at the end of each mount-map entry.@refill

The selectors currently implemented are:

@table @samp
@cindex arch, mount selector
@cindex Mount selector; arch
@cindex Selector; arch
@item arch
the machine architecture which was automatically determined at compile
time.  The architecture type can be displayed by running the command
@samp{amd -v}.  @xref{Supported Machine Architectures}.@refill

@item autodir
@cindex autodir, mount selector
@cindex Mount selector; autodir
@cindex Selector; autodir
the default directory under which to mount filesystems.  This may be
changed by the ``-a'' command line option.  See the @var{fs} option.

@item byte
@cindex byte, mount selector
@cindex Mount selector; byte
@cindex Selector; byte
the machine's byte ordering.  This is either @samp{little}, indicating
little-endian, or @samp{big}, indicating big-endian.  One possible use
is to share @samp{rwho} databases (@pxref{rwho servers}).  Another is to
share ndbm databases, however this use can be considered a courageous
juggling act.

@item cluster
@cindex cluster, mount selector
@cindex Mount selector; cluster
@cindex Selector; cluster
is provided as a hook for the name of the local cluster.  This can be
used to decide which servers to use for copies of replicated
filesystems.  @code{$@{cluster@}} defaults to the value of
@code{$@{domain@}} unless a different value is set with the ``-C''
command line option.

@item domain
@cindex domain, mount selector
@cindex Mount selector; domain
@cindex Selector; domain
the local domain name as specified by the ``-d'' command line option.
See @samp{host}.

@item host
@cindex host, mount selector
@cindex Mount selector; host
@cindex Selector; host
the local hostname as determined by @b{gethostname}(2).  If no domain
name was specified on the command line and the hostname contains a
period @samp{.} then the string before the period is used as the
host name, and the string after the period is assigned to
@code{$@{domain@}}.  For example, if the hostname is
@samp{styx.doc.ic.ac.uk} then @code{host} would be @samp{styx} and
@code{domain} would be @samp{doc.ic.ac.uk}.  @code{hostd} would be
@samp{styx.doc.ic.ac.uk}.@refill

@item hostd
@cindex hostd, mount selector
@cindex Mount selector; hostd
@cindex Selector; hostd
is @code{$@{host@}} and @code{$@{domain@}} concatenated with a
@samp{.} inserted between them if required.  If @code{$@{domain@}}
is an empty string then @code{$@{host@}} and @code{$@{hostd@}} will be
identical.

@item karch
@cindex karch, mount selector
@cindex Mount selector; karch
@cindex Selector; karch
is provided as a hook for the kernel architecture.  This is used on
SunOS 4, for example, to distinguish between different @samp{/usr/kvm}
volumes.  @code{$@{karch@}} defaults to the value of @code{$@{arch@}}
unless a different value is set with the ``-k'' command line option.

@item os
@cindex os, mount selector
@cindex Mount selector; os
@cindex Selector; os
the operating system.  Like the machine architecture, this is
automatically determined at compile time.  The operating system name can
be displayed by running the command @samp{amd -v}.  @xref{Supported
Operating Systems}.@refill

@end table

The following selectors are also provided.  Unlike the other selectors,
they vary for each lookup.  Note that when the name from the kernel is
expanded prior to a map lookup, these selectors are all defined as empty
strings.

@table @samp
@item key
@cindex key, mount selector
@cindex Mount selector; key
@cindex Selector; key
the name being resolved.  For example, if @file{/home} is an automount
point, then accessing @file{/home/foo} would set @code{$@{key@}} to the
string @samp{foo}.  The key is prefixed by the @var{pref} option set in
the parent mount point.  The default prefix is an empty string.  If the
prefix was @file{blah/} then @code{$@{key@}} would be set to
@file{blah/foo}.@refill

@item map
@cindex map, mount selector
@cindex Mount selector; map
@cindex Selector; map
the name of the mount map being used.

@item path
@cindex path, mount selector
@cindex Mount selector; path
@cindex Selector; path
the full pathname of the name being resolved.  For example
@file{/home/foo} in the example above.

@item wire
@cindex wire, mount selector
@cindex Mount selector; wire
@cindex Selector; wire
the name of the network to which the primary network interface is
attached.  If a symbolic name cannot be found in the networks or hosts
database then dotted IP address format is used.  This value is also
output by the ``-v'' option.

@end table

Selectors can be negated by using @samp{!=} instead of @samp{==}.  For
example to select a location on all non-Vax machines the selector
@samp{arch!=vax} would be used.

@node     Map Options,  , Selectors, Location Format
@comment  node-name,  next,  previous,  up
@subsection Map Options
@cindex Map options
@cindex Setting map options

Options are parsed concurrently with selectors.  The difference is that
when an option is seen the string following the @samp{:=} is
recorded for later use.  As a minimum the @var{type} option must be
specified.  Each filesystem type has other options which must also be
specified.  @xref{Filesystem Types}, for details on the filesystem
specific options.@refill

Superfluous option specifications are ignored and are not reported
as errors.

The following options apply to more than one filesystem type.

@menu
* delay Option::
* fs Option::
* opts Option::
* remopts Option::
* sublink Option::
* type Option::
@end menu

@node     delay Option, fs Option, Map Options, Map Options
@comment  node-name,  next,  previous,  up
@subsubsection delay Option
@cindex Setting a delay on a mount location
@cindex Delaying mounts from specific locations
@cindex Primary server
@cindex Secondary server
@cindex delay, mount option
@cindex Mount option; delay

The delay, in seconds, before an attempt will be made to mount from the current location.
Auxilliary data, such as network address, file handles and so on are computed
regardless of this value.

A delay can be used to implement the notion of primary and secondary file servers.
The secondary servers would have a delay of a few seconds,
thus giving the primary servers a chance to respond first.

@node     fs Option, opts Option, delay Option, Map Options
@comment  node-name,  next,  previous,  up
@subsubsection fs Option
@cindex Setting the local mount point
@cindex Overriding the default mount point
@cindex fs, mount option
@cindex Mount option; fs