4:55 a.m.
On 12/08/21 16:21, Richard W.M. Jones wrote:
On Wed, Dec 08, 2021 at 01:20:49PM +0100, Laszlo Ersek wrote:
> Add the "Nbdkit.get_disk_allocated" helper function, which calculates the
> number of allocated bytes in an output disk image, through a corresponding
> NBD server connection.
>
> Original "NBD.block_status" summation example code by Rich Jones
> (thanks!), chunking, metadata context simplification, and virt-v2v
> integration by myself. Chunking added because "NBD.block_status" is not
> 64-bit clean just yet (Eric Blake is working on it).
>
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2027598
> Signed-off-by: Laszlo Ersek <lersek(a)redhat.com>
> ---
> lib/nbdkit.mli | 8 ++++++
> lib/nbdkit.ml | 30 ++++++++++++++++++++
> 2 files changed, 38 insertions(+)
>
> diff --git a/lib/nbdkit.mli b/lib/nbdkit.mli
> index 825755e61dbe..03107dab109d 100644
> --- a/lib/nbdkit.mli
> +++ b/lib/nbdkit.mli
> @@ -117,3 +117,11 @@ val with_connect_unix : socket:string ->
> in [meta_contexts] requested (each of which is not necessarily supported by
> the server though). The connection is torn down either on normal return or
> if the function [f] throws an exception. *)
> +
> +val get_disk_allocated : dir:string -> disknr:int -> int64 option
> +(** Callable only in the finalization step. [get_disk_allocated dir disknr]
> + examines output disk [disknr] through the corresponding NBD server socket
> + that resides in [dir]. Returns the number of bytes allocated in the disk
> + image, according to the "base:allocation" metadata context. If the
context
> + is not supported by the NBD server behind the socket, the function returns
> + None. *)
This really is the right function, wrong module.
Since it only applies to output modules, the logical place is in
output/output.ml, where there are already a bunch of random helper
functions.
That's precisely where I put it first (given that we have "get_disks"
there too!). However, I proved simply unable to *call* the function,
from "lib/create_ovf.ml", in the next patch!
I tried "Output.get_disk_allocated", and I got a weird compilation
failure, even though the function was properly declared in "output.mli".
At that point, I couldn't decide if I was missing something about the
tricky "first class modules" thing, or if it was really a layering
violation to call an Output function from a *lower level* utility
function (such as "create_ovf" in "lib/create_ovf.ml").
I briefly considered that, whoever called create_ovf from the top level,
could *pass in* Output.get_disk_allocated to it, as a callback function.
But that seemed so horrendously ugly (and uncalled-for) that I dropped
the idea.
Yet another idea was to collect the actual sizes for all the output
disks in one go, separately, then pass in that list, to create_ovf. Then
in create_ovf, I would replace:
(* Iterate over the disks, adding them to the OVF document. *)
List.iteri (
fun i (size, image_uuid, vol_uuid) ->
...
) (List.combine3 sizes image_uuids vol_uuids)
with "combine4", adding a fourth component (the actual size) to the
tuple parameter of the nameless iterator function.
But that idea looked too complex as well.
Halp! :)
> diff --git a/lib/nbdkit.ml b/lib/nbdkit.ml
> index 43e0fc5c728a..e142c210c66f 100644
> --- a/lib/nbdkit.ml
> +++ b/lib/nbdkit.ml
> @@ -229,3 +229,33 @@ let with_connect_unix ~socket ~meta_contexts ~f =
> ~finally:(fun () -> NBD.shutdown nbd)
> )
> ~finally:(fun () -> NBD.close nbd)
> +
> +let get_disk_allocated ~dir ~disknr =
> + let socket = sprintf "%s/out%d" dir disknr
> + and alloc_ctx = "base:allocation" in
> + with_connect_unix ~socket ~meta_contexts:[alloc_ctx]
> + ~f:(fun nbd ->
> + if NBD.can_meta_context nbd alloc_ctx then (
> + (* Get the list of extents, using a 2GiB chunk size as hint. *)
> + let size = NBD.get_size nbd
> + and allocated = ref 0_L
> + and fetch_offset = ref 0_L in
> + while !fetch_offset < size do
> + let remaining = size -^ !fetch_offset in
> + let fetch_size = min 0x8000_0000_L remaining in
> + NBD.block_status nbd fetch_size !fetch_offset
> + (fun ctx offset entries err ->
> + assert (ctx = alloc_ctx);
> + for i = 0 to Array.length entries / 2 - 1 do
> + let len = Int64.of_int32 entries.(i * 2)
> + and typ = entries.(i * 2 + 1) in
> + if Int32.logand typ 1_l = 0_l then
> + allocated := !allocated +^ len;
> + fetch_offset := !fetch_offset +^ len
> + done;
> + 0
> + )
> + done;
> + Some !allocated
> + ) else None
> + )
But the function itself is fine.
Thanks!
Laszlo