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

[TobiasHunger]

Public bug reported:

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

** Affects: qemu
     Importance: Undecided
         Status: New

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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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