[Bug 1884831] Re: qemu-nbd fails to discard bigger chunks

[Eric Blake]

Let's get nbd.ko out of the picture.  The problem can be reproduced in
user space (here, where I built qemu-nbd to log trace messages to
stderr):

$ truncate --size=3G file
$ qemu-nbd -f raw file --trace=nbd_\*
$ nbdsh -u nbd://localhost:10810 -c 'h.trim(3*1024*1024*1024,0)'
Traceback (most recent call last):
  File "/usr/lib64/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib64/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/usr/lib64/python3.8/site-packages/nbd.py", line 1762, in <module>
    nbdsh.shell()
  File "/usr/lib64/python3.8/site-packages/nbdsh.py", line 100, in shell
    exec (c, d, d)
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.8/site-packages/nbd.py", line 1098, in trim
    return libnbdmod.trim (self._o, count, offset, flags)
nbd.Error: nbd_trim: trim: command failed: Input/output error (EIO)

and looking at the trace output from qemu-nbd, I see:
493771@1592948038.044141:nbd_negotiate_success Negotiation succeeded
493771@1592948038.044167:nbd_trip Reading request
493771@1592948038.044262:nbd_receive_request Got request: { magic = 0x25609513, 
.flags = 0x0, .type = 0x4, from = 0, len = 3221225472 }
493771@1592948038.044272:nbd_co_receive_request_decode_type Decoding type: 
handle = 1, type = 4 (trim)
493771@1592948038.044291:nbd_co_send_structured_error Send structured error 
reply: handle = 1, error = 5 (EIO), msg = 'discard failed'

so this is definitely a case of qemu as NBD server NOT honoring requests
between 2G and 4G.  I'll have a patch posted soon.

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https://bugs.launchpad.net/bugs/1884831

Title:
  qemu-nbd fails to discard bigger chunks

Status in QEMU:
  New

Bug description:
  This report is moved from systemd to here:
  https://github.com/systemd/systemd/issues/16242

  A qemu-nbd device reports that it can discard a lot of bytes:

  cat /sys/block/nbd0/queue/discard_max_bytes
  2199023255040

  And indeed, discard works with small images:

  $ qemu-img create -f qcow2 /tmp/image.img 2M
  $ sudo qemu-nbd --connect=/dev/nbd0 /tmp/image.img
  $ sudo blkdiscard /dev/nbd0

  but not for bigger ones (still smaller than discard_max_bytes):

  $ qemu-img create -f qcow2 /tmp/image.img 5G
  $ sudo qemu-nbd --connect=/dev/nbd0 /tmp/image.img
  $ sudo blkdiscard /dev/nbd0

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