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

[Eric Blake]

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