nbdkit ruby script=/path/to/plugin.rb [arguments...]
DESCRIPTION"nbdkit-ruby-plugin" is an embedded Ruby interpreter for nbdkit(1), allowing you to write nbdkit plugins in Ruby.
Broadly speaking, Ruby nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.
USING A RUBY NBDKIT PLUGINAssuming you have a Ruby script which is an nbdkit plugin, you run it like this:
nbdkit ruby script=/path/to/ruby.rb
You may have to add further "key=value" arguments to the command line. Read the Ruby script to see if it requires any. "script=..." must come first on the command line.
WRITING A RUBY NBDKIT PLUGINThere is an example Ruby nbdkit plugin called "example.rb" which ships with the nbdkit source.
To write a Ruby nbdkit plugin, you create a Ruby file which contains at least the following required functions:
def open(readonly) # see below end def get_size(h) # see below end def pread(h, count, offset) # see below end
Note that the subroutines must have those literal names (like "open"), because the C part looks up and calls those functions directly. You may want to include documentation and globals (eg. for storing global state). Any other top level statements are run when the script is loaded, just like ordinary Ruby.
The file does not need to include a "#!" (hash-bang) at the top, and does not need to be executable. In fact it's a good idea not to do that, because running the plugin directly as a Ruby script won't work.
EXCEPTIONSRuby callbacks should throw exceptions to indicate errors.
RUBY CALLBACKSThis just documents the arguments to the callbacks in Ruby, 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).
def config(key, value) # no return value end
There are no arguments or return value.
def open(readonly) # return handle end
You can return any non-nil Ruby value as the handle. It is passed back in subsequent calls.
def close(h) # no return value end
def get_size(h) # return the size of the disk end
def can_write(h) # return a boolean end
def can_flush(h) # return a boolean end
def is_rotational(h) # return a boolean end
def can_trim(h) # return a boolean end
def pread(h, count, offset) # construct a string of length count bytes and return it end
The body of your "pread" function should construct a string 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.
def pwrite(h, buf, offset) length = buf.length # 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 throw an exception.
def flush(h) # no return value end
def trim(h, count, offset) # no return value end
The body of your "trim" function should ``punch a hole'' in the backing store.
- Missing: "load" and "unload"
- These are not needed because you can just use ordinary Ruby constructs.
- Missing: "name", "version", "longname", "description", "config_help"
- These are not yet supported.
THREADSThe thread model for Ruby callbacks currently cannot be set from Ruby. It is hard-coded in the C part to "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS". This may change or be settable in future.
AUTHORSRichard W.M. Jones
COPYRIGHTCopyright (C) 2013-2016 Red Hat Inc.
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