nbdkit-lua-plugin - Man Page

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.

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).

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

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

These are not yet supported.

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.