nbdkit-ruby-plugin - nbdkit ruby plugin
nbdkit ruby /path/to/plugin.rb [arguments...]
The Ruby language is fundamentally broken when it comes to embedding in a program which uses pthreads. This means you may see random "stack overflows" when using this plugin on some versions of Ruby but not others.
For the whole sorry saga, see: https://redmine.ruby-lang.org/issues/2294
nbdkit-ruby-plugin
is an embedded Ruby interpreter for nbdkit(1), allowing you to write nbdkit plugins in Ruby.
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.
For an example plugin written in Ruby, see: https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/ruby/example.rb
Broadly speaking, Ruby nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.
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.
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).
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.
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
.
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)
.
load
and unload
These are not needed because you can just use ordinary Ruby constructs.
name
, version
, longname
, description
, config_help
, can_fua
, can_cache
, cache
These are not yet supported.
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.
The plugin.
Use nbdkit --dump-config
to find the location of $plugindir
.
nbdkit-ruby-plugin
first appeared in nbdkit 1.2.
nbdkit(1), nbdkit-plugin(3), ruby(1).
Eric Blake
Richard W.M. Jones
Copyright Red Hat
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