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

[Eric Blake <1884831@bugs.launchpad.net>]

Hmm, carrying on further, with the nbd-client connection, I'm seeing
that the kernel DID break things into two separate BLKDISCARD calls, as
seen from the nbdkit side of things:

# from the blkdiscard strace:
ioctl(3, BLKGETSIZE64, [5368709120])    = 0
ioctl(3, BLKSSZGET, [512])              = 0
ioctl(3, BLKDISCARD, [0, 5368709120])   = 0
# from the nbdkit debug log:
nbdkit: memory.0: debug: memory: trim count=4294966784 offset=0 fua=0
nbdkit: memory.3: debug: memory: trim count=1073742336 offset=4294966784 fua=0

I'm now comparing the set of ioctl calls made by nbd-client vs. qemu-nbd
to see what might explain the difference for why it worked with nbd-
client when the two different servers connect to the kernel nbd.ko
module.  In the meantime, since nbd-client worked but qemu-nbd did not,
it does look like this may be qemu's problem after all.


** Project changed: nbd => qemu

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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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