nbdkit-ruby-plugin3

If you have been given an nbdkit Ruby plugin

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

 nbdkit ruby /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.

Executable script

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

 #!/usr/sbin/nbdkit ruby

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 has access to the "Nbdkit" module, with the following singleton methods:

 Nbdkit.set_error(err)

Record "err" as the reason you are about to raise an exception. "err" should either be a class that defines an "Errno" constant (all of the subclasses of "SystemCallError" in module "Errno" have this property), an object that defines an "errno" method with no arguments (all instances of "SystemCallError" have this property), or an integer value corresponding to the usual errno values.

Exceptions

Ruby 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".

Ruby callbacks

This 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).

"dump_plugin"

(Optional)

There are no arguments or return value.

"config"

(Optional)

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

"config_complete"

(Optional)

There are no arguments or return value.

"open"

(Required)

 def open(readonly)
   # return handle
 end

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

"close"

(Optional)

 def close(h)
   # no return value
 end

"get_size"

(Required)

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

"can_write"

(Optional)

 def can_write(h)
   # return a boolean
 end

"can_flush"

(Optional)

 def can_flush(h)
   # return a boolean
 end

"is_rotational"

(Optional)

 def is_rotational(h)
   # return a boolean
 end

"can_trim"

(Optional)

 def can_trim(h)
   # return a boolean
 end

"pread"

(Required)

 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, optionally using "Nbdkit.set_error" first.

"pwrite"

(Optional)

 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, optionally using "Nbdkit.set_error" first.

"flush"

(Optional)

 def 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.

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
 end

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

Threads

The 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.