[Libguestfs] [PATCH v2] v2v/v2v.ml: Use larger request size for -o rhv-upload

[Richard W.M. Jones]

On Mon, Feb 14, 2022 at 04:52:17PM +0100, Laszlo Ersek wrote:
> On 02/14/22 14:01, Richard W.M. Jones wrote:
> > But nbdcopy needs to be reworked to make the input and output requests
> > separate, so that nbdcopy will coalesce and split blocks as it copies.
> > This is difficult.
> > 
> > Another problem I'm finding (eg
> > https://bugzilla.redhat.com/show_bug.cgi?id=2039255#c9) is that
> > performance of new virt-v2v is extremely specific to input and output
> > mode, and hardware and network configurations.  For reasons that I
> > don't fully understand.
> 
> How are the nbdcopy source and destination coupled with each other? From
> work I'd done a decade ago, I remember that connecting two
> network-oriented (UDP) processes with a small-buffer pipe between them
> caused very bad effects. Whenever either process was blocked on the
> network (or on a timer, for example), the pipe went immediately full or
> empty (dependent on the particular blocked process), which in turn
> blocked the other process almost immediately. So the mitigation for that
> was to create a simple local app, to be inserted between the two
> network-oriented processes in the pipeline, just to de-couple them from
> each other, and make sure that a write to the pipe, or a read from it,
> would effectively never block. (The app-in-the-middle did have a maximum
> buffer size, but it was configurable, so not a practical limitation; it
> could be multiple tens of MB if needed.)
> 
> If nbdcopy does some internal queueing (perhaps implicitly, i.e. by
> allowing multiple requests to be in flight at the same time), then
> seeing some stats on those "in real time" could be enlightening.

So the way it works at the moment is it's event driven.  Ignoring
extents to keep the description simple, we issue asynch read requests
(ie. nbd_aio_pread) and in the completion callbacks of those requests,
asynchronous write requests are started (ie. nbd_aio_pwrite).

https://gitlab.com/nbdkit/libnbd/-/blob/6725fa0e129f9a60d7b89707ef8604e0aeeeaf43/copy/multi-thread-copying.c#L372

There is a limit on the number of parallel requests in flight
(nbdcopy --requests, default 64).  This limits the implicit buffer to
max_requests * request_size.  That's 16MB in the default
configuration.  Quite small actually ...

https://gitlab.com/nbdkit/libnbd/-/blob/6725fa0e129f9a60d7b89707ef8604e0aeeeaf43/copy/multi-thread-copying.c#L239

I've managed to reproduce the problem locally now so I can try playing
with this limit.

Rich.

-- 
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