[Qemu-devel] [Bug 1474263] Re: "Image format was not specified" warning should be suppressed for the vvfat (and probably nbd) driver

[felix]

> I could actually see the use of non-raw over NBD.  We support nested
> protocols (where you can use qcow2->qcow2->file), that is, where a file
> contains a qcow2 file whose contents are themselves a qcow2 image.
> (Perhaps useful in nested guests, where the outer qcow2 layer serves a
> disk to an L0 guest, which in turn uses the inner layer to present a
> disk to an L1 guest).  In such a case, opening just one layer of qcow2
> for service over NBD will expose the inner qcow2 image, and connecting
> qemu as an NBD client with format=raw will directly manipulate the qcow2
> data seen by the L0 guest, while connecting as an NBD client with
> format=qcow2 will see the raw data seen by the L1 guest.

Seems like an academic exercise, really. But if this use case is
practical, I believe three levels of wrapping is just as useful, and the
only way to work with that one is to run two or three instances of qemu-
nbd. With more layers, the set-up quickly becomes tangled and
unmanageable.

And I still doubt anyone would actually want to create a set-up like
this. It seems incredibly wasteful (but then, so is virtualisation in
general, so maybe that isn't an issue) and doesn't seem to accomplish
anything that couldn't be done with just one layer of wrapping.

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https://bugs.launchpad.net/bugs/1474263

Title:
  "Image format was not specified" warning should be suppressed for the
  vvfat (and probably nbd) driver

Status in QEMU:
  New

Bug description:
  Running

  qemu -drive file.driver=vvfat,file.dir=.

  displays

  WARNING: Image format was not specified for 'json:{"dir": ".", "driver": 
"vvfat"}' and probing guessed raw.
           Automatically detecting the format is dangerous for raw images, 
write operations on block 0 will be restricted.
           Specify the 'raw' format explicitly to remove the restrictions.

  However, since "images" provided by the vvfat driver are always raw
  (and the first sector isn't writeable in any case), this warning is
  superfluous and should not be displayed.

  A similar warning is displayed for NBD devices; I suspect it should be
  also disabled for similar reasons, but I'm not sure if serving non-raw
  images is actually a violation of the protocol. qemu-nbd translates
  them to raw images, for what it's worth, even if it may be suppressed
  with -f raw.

  Noticed on 2.3.0; the code that causes this behaviour is still
  apparently present in today's git master
  (f3a1b5068cea303a55e2a21a97e66d057eaae638). Speaking of versions: you
  may want to update the copyright notice that qemu -version displays.

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