Re: [Qemu-devel] [Nbd] [PATCH] proto: add 'shift' extension.

[Wouter Verhelst]

On Mon, Sep 26, 2016 at 03:21:46PM -0500, Eric Blake wrote:
> I'd much rather support a single flag that says to zero the entire disk
> than to introduce stateful variable-amount shifting.

That's almost exactly the opposite of what I said :)

Now, I don't feel very strong either way, but what matters to me is:

- NBD is a simple, easy to understand protocol; that is a feature, and
  so it should remain that way.
- Every time we add another option, flag, or command, we make the
  protocol slightly more complex, which is counter to that goal.
- Adding a command with a single use case (i.e., a "wipe the whole
  device" command) seems like it would not see much use, except perhaps
  in the use case that Virtuozzo is thinking about. In other words, it
  makes things slightly more complex for little benefit.

I thought a negotiated shift size could be creatively used for other
things beyond just "wipe the whole disk" commands, and that it might be
elegant in that way. On the other hand, I recognize that adding state in
that manner also complicates the protocol in that an observer which sees
only part of the traffic may not understand what's going on anymore.

So let's just say that an NBD_CMD_FLAG_SHIFT would:
- Left-shift the size by 16 bits; no more, no less
  - 2^32-1 is too large a granularity for this to be useful beyond "wipe
    whole disk" commands; 2^16-1 (65535) seems like a more useful
    granularity.
  - This allows for a maximum number of 2^48-1 bytes (one byte shy of
    256 tebibytes) to be affected by a single command, which seems
    sufficient for the given purpose.
  - If someone really wants to wipe 2^64-1 bytes (i.e., 16 exbibytes),
    they are probably using the wrong tools.
- Be only valid for commands that don't send or expect data to be sent
  out over the wire.
  - currently TRIM and WRITE_ZEROES, but not READ or WRITE.

Thoughts?

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12