If you have been given an nbdkit sh plugin¶

Assuming you have a shell script which is an nbdkit plugin, you run it like this:

 nbdkit sh /path/to/script

You may have to add further "key=value" arguments to the command line. The script must be executable ("chmod +x").

Inline shell scripts¶

It is also possible to write a shell script plugin "inline" using "-" as the name of the script, like this:

 nbdkit sh - <<'EOF'
   case "$1" in
     get_size) echo 1M ;;
     pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
     *) exit 2 ;;
   esac
 EOF

By default the inline script runs under /bin/sh. You can add a shebang ("#!") to use other scripting languages. Of course, reading an inline script from stdin is incompatible with the -s (--single) mode of nbdkit that connects a client on stdin.

Programming model¶

This plugin has a simple programming model: For every plugin method that needs to be called, the external script is invoked with parameters describing the method and its arguments. The first parameter is always the method name. For example:

 /path/to/script config file disk.img
                   │      │   │
                   │      │   └─ value ($3)
                   │      └── key ($2)
               method ($1)

 /path/to/script pread <handle> <count> <offset>
                   │       │       │       │
                   │       │       │       └─ offset in bytes ($4)
                   │       │       └── request size in bytes ($3)
               method ($1) └── handle ($2) ─ see "Handles" below

Scripts should ignore extra parameters that they don't understand since we may add new parameters in future.

Exit codes¶

The script should exit with specific exit codes:

0

The method was executed successfully.

1 and 16-255

There was an error. The script may print on stderr an errno name, optionally followed by whitespace and a message, for example:

 echo 'ENOSPC Out of space' >&2
 exit 1
    

or if you don't need the log message:

 echo ENOSPC >&2
 exit 1
    

If the script doesn't print anything or the output cannot be parsed then nbdkit assumes error "EIO". Note that output to stderr is ignored if the command succeeds, so it is acceptable to output a potential error message prefix prior to attempting a command which will add further details if a failure occurs.

2

The requested method is not supported by the script.

3

For methods which return booleans, this code indicates false.

4 and 5

Triggers a call to the C function "nbdkit_shutdown", which requests an asynchronous exit of the nbdkit server (disconnecting all clients). The client will usually get a response before shutdown is complete (although this is racy); so once the shutdown is requested, code 4 then behaves like code 0 (stderr is ignored, and the server tries to return success), and code 5 behaves like code 1 (the server tries to return an error to the client parsed from stderr, although a missing error defaults to "ESHUTDOWN" instead of "EIO").

6

Triggers a call to the C function "nbdkit_disconnect" with "force" set to true, which requests an abrupt disconnect of the current client. The contents of stderr are irrelevant with this status, since the client will not get a response.

7 and 8

Triggers a call to the C function "nbdkit_disconnect" with "force" set to false, which requests a soft disconnect of the current client (future client requests are rejected with "ESHUTDOWN" without calling into the plugin, but current requests may complete). Since the client will likely get the response to this command, code 7 then behaves like code 0 (stderr is ignored, and the server tries to return success), and code 8 behaves like code 1 (the server tries to return an error to the client parsed from stderr, although a missing error defaults to "ESHUTDOWN" instead of "EIO").

9-15

These exit codes are reserved for future use. Note that versions of nbdkit < 1.34 documented that codes 8 through 15 behaved like code 1; although it is unlikely that many scripts relied on this similarity in practice.

In nbdkit > 1.34, it is possible to probe whether additional exit codes have been assigned meaning, by looking for the line max_known_status= in the output of nbdkit --dump-plugin sh. If this line is not present, exit codes 4 and above behave like status 1.

Temporary directory¶

A fresh script is invoked for each method call (ie. scripts are stateless), so if the script needs to store state it has to store it somewhere in the filesystem in a format and location which is left up to the author of the script.

However nbdkit helps by creating a randomly named, empty directory for the script. This directory persists for the lifetime of nbdkit and is deleted when nbdkit exits. The name of the directory is passed to each script invocation in the $tmpdir environment variable.

Handles¶

Handles are arbitrary strings, but it is best to limit them to short alphanumeric strings.

Per-connection state

The temporary directory described above can be used for state for the lifetime of the nbdkit instance (across multiple connections). If you want to store state per connection then one way to do it is to create a randomly named subdirectory under the temporary directory:

 case "$1" in
   ...
   open)
     mktemp -d $tmpdir/handle-XXXXXX ;;

The handle will be the subdirectory name, returned to the script as $2 in all connected calls (eg. "pread", "get_size"). You can delete the subdirectory explicitly in "close":

 case "$1" in
   ...
   close)
     rm -rf "$2" ;;

or rely on nbdkit deleting the whole temporary directory including all per-handle subdirectories when it exits.

Performance¶

This plugin has to fork on every request, so performance will never be great. For best performance, consider using the nbdkit-plugin(3) API directly. Having said that, if you have a sh plugin and want to improve performance then the following tips may help:

Relax the thread model.

The default "thread_model" is "serialize_all_requests" meaning that two instances of the script can never be running at the same time. This is safe but slow. If your script is safe to be called in parallel, set this to "parallel".

Implement the "zero" method.

If the "zero" method is not implemented then nbdkit will fall back to using "pwrite" which is considerably slower because nbdkit has to send blocks of zeroes to the script.

You don't have to write shell scripts.

This plugin can run any external binary, not only shell scripts. You should get more performance by rewriting the shell script as a program in a compiled language.

Methods¶

This just documents the arguments to the script corresponding to each plugin method, and any way that they differ from the C callbacks. In all other respects they work the same way as the C callbacks, so you should go and read nbdkit-plugin(3).

"load"

 /path/to/script load
    

"unload"

 /path/to/script unload
    

This is called just before nbdkit exits. Errors from this method are ignored.

"dump_plugin"

 /path/to/script dump_plugin
    

"config"

 /path/to/script config <key> <value>
    

"config_complete"

 /path/to/script config_complete
    

"magic_config_key"

 /path/to/script magic_config_key
    

If a magic config key is needed, this should echo it to stdout. See "Magic parameters" in nbdkit(1).

"thread_model"

 /path/to/script thread_model
    

On success this should print the desired thread model of the script, one of "serialize_connections", "serialize_all_requests", "serialize_requests", or "parallel".

This method is not required; if omitted, then the plugin will be executed under the safe "serialize_all_requests" model. However, this means that this method must be provided if you want to use the "parallel" or "serialize_requests" model. Even then your request may be restricted for other reasons; look for "thread_model" in the output of "nbdkit --dump-plugin sh script" to see what actually gets selected.

If an error occurs, the script should output an error message and exit with status 1; unrecognized output is ignored.

"get_ready"

 /path/to/script get_ready
    

"after_fork"

 /path/to/script after_fork
    

"preconnect"

 /path/to/script preconnect <readonly>
    

"list_exports"

 /path/to/script list_exports <readonly> <tls>
    

The "readonly" and "tls" parameters will be "true" or "false".

The first line of output informs nbdkit how to parse the rest of the output, the remaining lines then supply the inputs of the C "nbdkit_add_export" function (see nbdkit-plugin(3)), as follows:

"default_export"

 /path/to/script default_export <readonly> <tls>
    

The "readonly" and "tls" parameters will be "true" or "false".

On success this should print a name on stdout to use in place of the default export "", then exit with code 0. For convenience, the output can be any of the list forms recognized by "list_exports", in which case the first listed export name is used, and where an empty list uses "". Given the current set of recognized export lists, it is not possible for the resulting name to include a newline.

This method is not required; if it is absent, the default export name will be the empty string, "".

"open"

 /path/to/script open <readonly> <exportname> <tls>
    

The "readonly" parameter will be "true" or "false". The "exportname" parameter, if present, is the export name passed to the server from the client. The "tls" parameter, if present, will be "true" or "false" depending on whether the client is using TLS.

On success this should print the handle (any string) on stdout and exit with code 0. If the handle ends with a newline character then the newline is removed.

Unlike C plugins, this method is not required. If omitted then the handle will be "" (empty string).

"close"

 /path/to/script close <handle>
    

"export_description"

 /path/to/script export_description <handle>
    

The script should print a human-readable description of the disk image on stdout. If the description ends with a newline character then the newline is removed.

This method is not required; if it is absent, no export description will be provided to the client.

"get_size"

 /path/to/script get_size <handle>
    

The script should print the size of the disk image on stdout. You can print the size in bytes, or use any format understood by "nbdkit_parse_size" such as "1M" (see "PARSING SIZE PARAMETERS" in nbdkit-plugin(3)).

This method is required.

"block_size"

 /path/to/script block_size <handle>
    

This script should print three numbers on stdout, separated by whitespace. These are (in order) the minimum block size, the preferred block size, and the maximum block size. You can print the sizes in bytes or use any format understood by "nbdkit_parse_size" such as "1M" (see "PARSING SIZE PARAMETERS" in nbdkit-plugin(3)).

"can_write"

"can_flush"

"can_trim"

"can_zero"

"can_extents"

Unlike in other languages, you must provide the "can_*" methods otherwise they are assumed to all return false and your "pwrite", "flush", "trim", "zero" and "extents" methods will never be called. The reason for this is obscure: In other languages we can detect if (eg) a "pwrite" method is defined and synthesize an appropriate response if no actual "can_write" method is defined. However detecting if methods are present without running them is not possible with this plugin.

 /path/to/script can_write <handle>
 /path/to/script can_flush <handle>
 /path/to/script can_trim <handle>
 /path/to/script can_zero <handle>
 /path/to/script can_extents <handle>
    

The script should exit with code 0 for true or code 3 for false.

"is_rotational"

"can_fast_zero"

 /path/to/script is_rotational <handle>
 /path/to/script can_fast_zero <handle>
    

The script should exit with code 0 for true or code 3 for false.

"can_fua"

"can_cache"

 /path/to/script can_fua <handle>
 /path/to/script can_cache <handle>
    

These control Forced Unit Access (FUA) and caching behaviour of the core server.

Unlike the other "can_*" callbacks, these two are not a boolean. They must print either "none", "emulate" or "native" to stdout. The meaning of these is described in nbdkit-plugin(3). Furthermore, you must provide a "can_cache" method if you desire the "cache" callback to be utilized, similar to the reasoning behind requiring "can_write" to utilize "pwrite".

"can_multi_conn"

 /path/to/script can_multi_conn <handle>
    

The script should exit with code 0 for true or code 3 for false.

"pread"

 /path/to/script pread <handle> <count> <offset>
    

The script should print the requested binary data on stdout. Exactly "count" bytes must be printed.

This method is required.

"pwrite"

 /path/to/script pwrite <handle> <count> <offset> <flags>
    

The script should read the binary data to be written from stdin.

The "flags" parameter can be an empty string or "fua". In the future, a comma-separated list of flags may be present.

Unlike in other languages, if you provide a "pwrite" method you must also provide a "can_write" method which exits with code 0 (true).

"flush"

 /path/to/script flush <handle>
    

Unlike in other languages, if you provide a "flush" method you must also provide a "can_flush" method which exits with code 0 (true).

"trim"

 /path/to/script trim <handle> <count> <offset> <flags>
    

The "flags" parameter can be an empty string or "fua". In the future, a comma-separated list of flags may be present.

Unlike in other languages, if you provide a "trim" method you must also provide a "can_trim" method which exits with code 0 (true).

"zero"

 /path/to/script zero <handle> <count> <offset> <flags>
    

The "flags" parameter can be an empty string or a comma-separated list of the flags: "fua", "may_trim", and "fast" (eg. "", "fua", "fua,may_trim,fast" are some of the 8 possible values).

Unlike in other languages, if you provide a "zero" method you must also provide a "can_zero" method which exits with code 0 (true).

To trigger a fallback to "pwrite" on a normal zero request, or to respond quickly to the "fast" flag that a specific zero request is no faster than a corresponding write, the script must output "ENOTSUP" or "EOPNOTSUPP" to stderr (possibly followed by a description of the problem) before exiting with code 1 (failure).

"extents"

 /path/to/script extents <handle> <count> <offset> <flags>
    

The "flags" parameter can be an empty string or "req_one".

This must print, one per line on stdout, a list of one or more extents in the format:

 offset length type
    

which correspond to the inputs of the C "nbdkit_add_extent" function (see nbdkit-plugin(3)). The "offset" and "length" fields may use any format understood by "nbdkit_parse_size". The optional "type" field may be an integer, missing (same as 0), or a comma-separated list of the words "hole" and "zero". An example of a valid set of extents covering a "10M" disk where the first megabyte only is allocated data:

 0  1M
 1M 9M  hole,zero
    

Unlike in other languages, if you provide an "extents" method you must also provide a "can_extents" method which exits with code 0 (true).

"cache"

 /path/to/script cache <handle> <count> <offset>
    

Unlike in other languages, if you provide a "cache" method you must also provide a "can_cache" method which prints "native" and exits with code 0 (true).

Missing callbacks¶

Missing: "name", "version", "longname", "description", "config_help"

These are not yet supported.