If you have been given an nbdkit Python plugin¶

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

 nbdkit python /path/to/plugin.py

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

Python 2 and Python 3¶

The Python plugin has to be compiled for either Python 2 or Python 3 when building nbdkit. You can set the "PYTHON" environment variable to the desired interpreter, otherwise nbdkit will use the interpreter called "python" on the current $PATH. For example:

 PYTHON=/usr/bin/python3 ./configure

To find out which version the Python plugin was compiled for, use the --dump-plugin option, eg:

 $ nbdkit python --dump-plugin
 ...
 python_version=3.7.0
 python_pep_384_abi_version=3

Executable script¶

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

 #!/usr/sbin/nbdkit python

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).

Methods¶

Your script may use "import nbdkit" to have access to the following methods in the "nbdkit" module:

 nbdkit.set_error(err)

Record "err" as the reason you are about to throw an exception. "err" should correspond to usual errno values, where it may help to "import errno".

Exceptions¶

Python callbacks should throw exceptions to indicate errors. Remember to use "nbdkit.set_error" if you need to control which error is sent back to the client; if omitted, the client will see an error of "EIO".

Python callbacks¶

This just documents the arguments to the callbacks in Python, 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)

 def config(key, value):
   # no return value
    

"config_complete"

(Optional)

There are no arguments or return value.

"open"

(Required)

 def open(readonly):
   # return handle
    

You can return any non-NULL Python value as the handle. It is passed back in subsequent calls.

"close"

(Optional)

 def close(h):
   # no return value
    

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)

 def get_size(h):
   # return the size of the disk
    

"can_write"

(Optional)

 def can_write(h):
   # return a boolean
    

"can_flush"

(Optional)

 def can_flush(h):
   # return a boolean
    

"is_rotational"

(Optional)

 def is_rotational(h):
   # return a boolean
    

"can_trim"

(Optional)

 def can_trim(h):
   # return a boolean
    

"pread"

(Required)

 def pread(h, count, offset):
   # construct a bytearray of length count bytes and return it
    

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 throw an exception, optionally using "nbdkit.set_error" first.

"pwrite"

(Optional)

 def pwrite(h, buf, offset):
   length = len (buf)
   # no return value
    

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 throw an exception,
optionally using "nbdkit.set_error" first.

"flush"

(Optional)

 def flush(h):
   # no return value
    

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

If the flush fails, your function should throw an exception, optionally using "nbdkit.set_error" first.

"trim"

(Optional)

 def trim(h, count, offset):
   # no return value
    

The body of your "trim" function should "punch a hole" in the backing store. If the trim fails, your function should throw an exception, optionally using "nbdkit.set_error" first.

"zero"

(Optional)

 def zero(h, count, offset, may_trim):
   # no return value
    

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 throw an exception, optionally using "nbdkit.set_error" first. In particular, if you would like to automatically fall back to "pwrite" (perhaps because there is nothing to optimize if "may_trim" is false), use "nbdkit.set_error(errno.EOPNOTSUPP)".

Missing callbacks¶

Missing: "load" and "unload"

These are not needed because you can just use ordinary Python constructs.

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

These are not yet supported.

Threads¶

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