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 versions

In nbdkit ≤ 1.14, either Python 2 or 3 could be used. It was selected at compile time by either:

 ./configure

which selected the version of Python by looking at the "python" interpreter found on the $PATH. Or:

 ./configure PYTHON=/usr/bin/python3

which allowed you to select a different interpreter and hence a different version of Python.

nbdkit ≥ 1.16 drops all support for Python 2, since Python 2 has reached its end of life.

The new behaviour is that "./configure" looks for "python3" or "python" (in that order) on the $PATH. It will fail if the first interpreter it finds is a Python 2 interpreter. You may also still choose a Python interpreter by setting the "PYTHON" variable at configure time as above.

If you wish to continue using nbdkit plugins written in Python 2 then you must use nbdkit ≤ 1.14, but we would advise you to update your plugins.

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

API versions

The nbdkit API has evolved and new versions are released periodically. To ensure backwards compatibility plugins have to opt in to the new version. From Python you do this by declaring a constant in your module:

 API_VERSION = 2

(where 2 is the latest version at the time this documentation was written). All newly written Python modules must have this constant.

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.debug(msg)"

Send a debug message to stderr or syslog if verbose messages are enabled.

"nbdkit.export_name()"

Return the export name negotiated with the client as a Unicode string. Note this should not be trusted because the client can send whatever it wants.

"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.

"thread_model"

(Optional, nbdkit ≥ 1.22)

 def thread_model():
   return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS
    

See "Threads" below.

"get_ready"

(Optional)

There are no arguments or return value.

"list_exports"

(Optional)

 def list_exports(readonly, is_tls):
   # return an iterable object (eg. list) of
   # (name, description) tuples or bare names:
   return [ (name1, desc1), name2, (name3, desc3), ... ]
    

"default_export"

(Optional)

 def default_export(readonly, is_tls):
   # return a string
   return "name"
    

"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.

"export_description"

(Optional)

 def export_description(h):
   # return a string
   return "description"
    

"get_size"

(Required)

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

"is_rotational"

(Optional)

 def is_rotational(h):
   # return a boolean
    

"can_multi_conn"

(Optional)

 def can_multi_conn(h):
   # return a boolean
    

"can_write"

(Optional)

 def can_write(h):
   # return a boolean
    

"can_flush"

(Optional)

 def can_flush(h):
   # return a boolean
    

"can_trim"

(Optional)

 def can_trim(h):
   # return a boolean
    

"can_zero"

(Optional)

 def can_zero(h):
   # return a boolean
    

"can_fast_zero"

(Optional)

 def can_fast_zero(h):
   # return a boolean
    

"can_fua"

(Optional)

 def can_fua(h):
   # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
   # or nbdkit.FUA_NATIVE
    

"can_cache"

(Optional)

 def can_cache(h):
   # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
   # or nbdkit.CACHE_NATIVE
    

"can_extents"

(Optional)

 def can_extents(h):
   # return a boolean
    

"pread"

(Required)

 def pread(h, buf, offset, flags):
   # read into the buffer
    

The body of your "pread" function should read exactly "len(buf)" bytes of data starting at disk "offset" and write it into the buffer "buf". "flags" is always 0.

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, flags):
   length = len(buf)
   # no return value
    

The body of your "pwrite" function should write the buffer "buf" to the disk. You should write "count" bytes to the disk starting at "offset". "flags" may contain "nbdkit.FLAG_FUA".

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, flags):
   # no return value
    

The body of your "flush" function should do a sync(2) or fdatasync(2) or equivalent on the backing store. "flags" is always 0.

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

"trim"

(Optional)

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

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

"zero"

(Optional)

 def zero(h, count, offset, flags):
   # 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. "flags" is a bitmask which may include "nbdkit.FLAG_MAY_TRIM", "nbdkit.FLAG_FUA", "nbdkit.FLAG_FAST_ZERO".

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 "flags & nbdkit.FLAG_MAY_TRIM" is false), use "nbdkit.set_error(errno.EOPNOTSUPP)".

"cache"

(Optional)

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

The body of your "cache" function should prefetch data in the indicated range.

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

"extents"

(Optional)

 def extents(h, count, offset, flags):
   # return an iterable object (eg. list) of
   # (offset, length, type) tuples:
   return [ (off1, len1, type1), (off2, len2, type2), ... ]
    

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", "magic_config_key".

These are not yet supported.

Threads

The thread model for Python callbacks defaults to "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS". Since nbdkit 1.22 it is possible to set this by implementing a "thread_model" function which returns one of the constants "nbdkit.THREAD_MODEL_*".

The Python Global Interpreter Lock (GIL) usually means that you cannot execute Python code in parallel, but Python code which calls into libraries which block (eg. to make HTTP requests) might be executed in parallel.