Re: [PATCH manager v3] fix #4503: ui: qemu: add detect zeroes disk option

["Lukas Sichert" <l.sichert@proxmox.com>]

On 2026-06-22 10:51, Daniel Kral <d.kral@proxmox.com> wrote:

> On Fri Jun 19, 2026 at 1:01 PM CEST, David Riley wrote:
>> On 6/18/26 1:49 PM, Lukas Sichert wrote:
>>> When discard is enabled, qemu-server defaults to 'detect-zeroes=unmap'.
>>> This can have a significant performance impact for zero writes on some
>>> setups, while users might still want to keep guest discard/TRIM support
>>> enabled.
>>>
>>> Allow configuring the option independently so 'discard' can stay enabled
>>> while zero detection is disabled. Persist the checkbox state explicitly,
>>> as the backend treats an absent value as default-enabled behavior,
>>> resulting in 'detect-zeroes=unmap' when 'discard' is enabled.
>>
>> Thanks for submitting this patch.
>>
>> While it clearly addresses the performance issues reported in #4503
>> [0], I was reading through the Bugzilla thread and noticed a request
>> to support the third QEMU state for detect_zeroes [1]. Currently, the
>> underlying parameter supports off, on, and unmap [2].
>>
>> Since we are already modifying this configuration in the UI, it might
>> make sense to fully expose all three options to the user rather than
>> just a binary toggle as there is a difference between "on" and
>> "unmap".
>
> This would need adapting the detect_zeroes option and parsing logic
> (it's only boolean right now) so it can take any of the 3 valid values
> by QEMU and be backwards compatible to the old values in our config.
>
> This should probably also check the constraint that detect-zeroes=unmap
> can only be set if discard=unmap, but the error message seems already be
> quite clear for that [0].

Already in src/PVE/QemuServer/Drive.pm in the qemu-server repo, this is
checked and set accordingly.

Since detect-zeroes=unmap is essentially an extension of
detect-zeroes=on [1], and detect-zeroes=unmap is currently the default
when discard is enabled, I would keep this patch as is to preserve
backward compatibility.

[1]: https://www.qemu.org/docs/master/interop/qemu-storage-daemon-qmp-ref.html#enum-QSD-block-core.BlockdevDetectZeroesOptions

> [0] https://gitlab.com/qemu-project/qemu/-/blob/master/block.c?ref_type=heads#L1120
>
>>
>> [0] https://bugzilla.proxmox.com/show_bug.cgi?id=4503
>> [1] https://bugzilla.proxmox.com/show_bug.cgi?id=4503#c8
>> [2] https://www.qemu.org/docs/master/interop/qemu-storage-daemon-qmp-ref.html#enum-QSD-block-core.BlockdevDetectZeroesOptions
>>
>>> Link: https://bugzilla.proxmox.com/show_bug.cgi?id=4503
>>> Signed-off-by: Lukas Sichert <l.sichert@proxmox.com>
>>> ---
>>>
>>> Notes:
>>>      changes from v2 to v3 (thanks @Fabian):
>>>      - add a comment explaining why this property is handled differently from
>>>        the others
>>>      
>>>      changes from v1 to v2:
>>>      - added bug-numer to commit header
>>>      - appended bug-link to commit body
>>>
>>>   www/manager6/qemu/HDEdit.js | 10 ++++++++++
>>>   1 file changed, 10 insertions(+)
>>>
>>> [...]
>>>