If you have been given an nbdkit Lua plugin

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

 nbdkit lua /path/to/plugin.lua

You may have to add further "key=value" arguments to the command line. Read the Lua script to see if it requires any.

Executable script

If you want you can make the script executable and include a "shebang" at the top:

 #!/usr/sbin/nbdkit lua

See also "Shebang scripts" in nbdkit(1).

These scripts can also be installed in the $plugindir. See "WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES" in nbdkit-plugin(3).

Errors

Lua plugin methods can indicate an error by calling "error" or "assert". The error message will contain the method name, filename and line number where the error occurred, eg:

 error ("could not open " .. filename)
 --> nbdkit: error: open: myplugin.lua:123: could not open disk.img

Lua callbacks

This just documents the arguments to the callbacks in Lua, 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).

"dump_plugin"

(Optional)

There are no arguments or return value.

"config"

(Optional)

 function config (key, value)
     -- No return value.
 end
    

"config_complete"

(Optional)

There are no arguments or return value.

"open"

(Required)

 function open (readonly)
     local handle
     handle=...
     return handle
 end
    

The "readonly" flag is a boolean.

You can return any Lua string or object as the handle. It is passed back to subsequent calls.

"close"

(Optional)

 function close (h)
     -- No return value
 end
    

After "close" returns, the reference count of the handle is decremented in the C part, which usually means that the handle and its contents will be garbage collected.

"get_size"

(Required)

 function get_size (h)
     local size
     size= .. the size of the disk ..
     return size
 end
    

This returns the size of the disk.

"can_write"

(Optional)

 function can_write (h)
     return bool
 end
    

Return a boolean indicating whether the disk is writable.

"can_flush"

(Optional)

 function can_flush (h)
     return bool
 end
    

Return a boolean indicating whether flush can be performed.

"is_rotational"

(Optional)

 function is_rotational (h)
     return bool
 end
    

Return a boolean indicating whether the disk is rotational.

"can_trim"

(Optional)

 function can_trim (h)
     return bool
 end
    

Return a boolean indicating whether trim/discard can be performed.

"pread"

(Required)

 function pread (h, count, offset)
    -- Construct a buffer of length count bytes and return it.
    return buf
 end
    

The body of your "pread" function should construct a buffer of length (at least) "count" bytes. You should read "count" bytes from the disk starting at "offset".

NBD only supports whole reads, so your function should try to read the whole region (perhaps requiring a loop). If the read fails or is partial, your function should call "error".

"pwrite"

(Optional)

 function pwrite (h, buf, offset)
    -- No return value
 end
    

The body of your "pwrite" function should write the "buf" string to the disk. You should write "count" bytes to the disk starting at "offset".

NBD only supports whole writes, so your function should try to write the whole region (perhaps requiring a loop). If the write fails or is partial, your function should call "error".

"flush"

(Optional)

 function flush (h)
     -- No return value
 end
    

The body of your "flush" function should do a sync(2) or fdatasync(2) or equivalent on the backing store.

"trim"

(Optional)

 function trim (h, count, offset)
     -- No return value
 end
    

The body of your "trim" function should "punch a hole" in the backing store.

"zero"

(Optional)

 function zero (h, count, offset, may_trim)
    -- No return value
 end
    

The body of your "zero" function should ensure that "count" bytes of the disk, starting at "offset", will read back as zero. If "may_trim" is true, the operation may be optimized as a trim as long as subsequent reads see zeroes.

NBD only supports whole writes, so your function should try to write the whole region (perhaps requiring a loop). If the write fails or is partial, your function should call "error".

Missing callbacks

Missing: "load", "unload", "name", "version", "longname", "description", "config_help", "can_zero", "can_fua", "can_cache", "cache"

These are not yet supported.

Threads

The thread model for Lua callbacks currently cannot be set from Lua. It is hard-coded in the C part to "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS". This may change or be settable in future.