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

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

On 6/23/20 3:19 PM, TobiasHunger wrote:
> 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

That smells fishy.  It is 0xffffffff * 512.  But in reality, the NBD 
protocol is (currently) capped at 32 bits, so it cannot handle any 
request 4G or larger.

It is not qemu-nbd that populates 
/sys/block/nbd0/queue/discard_max_bytes, but the kernel.  Are you sure 
this is not a bug in the kernel's nbd.ko module, where it may be the 
case that it is reporting -1 as a 32-bit value which then gets 
mistakenly turned into a faulty advertisement?  Can you tweak your 
software to behave as if /dev/nbd0 had a discard_max_bytes of 0xfffff000 
instead?

In fact, to prove the bug is in the kernel's nbd.ko and not in qemu-nbd, 
I created an NBD server using nbdkit:

# modprobe nbd
# nbdkit memory 5G
# nbd-client -b 512 localhost /dev/nbd0
# cat /sys/block/nbd0/queue/discard_max_bytes
2199023255040

Same answer, different nbd server.  So it's not qemu's fault.

> 
> 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
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


** Bug watch added: github.com/systemd/systemd/issues #16242
   https://github.com/systemd/systemd/issues/16242

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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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