Re: Format type of qemu NVMe virtual drive reverted back to its default (512 bytes block size) after performing hot plugout/plugin operation on that drive.

[Klaus Jensen <its@irrelevant.dk>]

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On Jul 25 16:53, Ashutosh Sharma wrote:
> Hi,
> 
> I have a virtual system created using qemu 7.2. In that system, I
> attached/hot plugged a virtual NVMe drive. This drive had a default
> block size of 512 bytes.
> 
> admin@node-3:~$ sudo nvme list
> Node                  SN                   Model
>              Namespace Usage                      Format           FW
> Rev
> --------------------- --------------------
> ---------------------------------------- ---------
> -------------------------- ---------------- --------
> /dev/nvme0n1          ashudev-6f34a1cf_13  QEMU NVMe Ctrl
>              1          34.36  GB /  34.36  GB    512   B +  0 B
> 7.1.92
> 
> After that, I formatted this drive with 4k block size and it formatted
> successfully.
> 
> admin@node-3:~$ sudo nvme format /dev/nvme0n1 -f --lbaf 4
> Success formatting namespace:1
> admin@node-3:~$
> admin@node-3:~$ sudo nvme list
> Node                  SN                   Model
>              Namespace Usage                      Format           FW
> Rev
> --------------------- --------------------
> ---------------------------------------- ---------
> -------------------------- ---------------- --------
> /dev/nvme0n1          ashudev-6f34a1cf_13  QEMU NVMe Ctrl
>              1          34.36  GB /  34.36  GB      4 KiB +  0 B
> 7.1.92
> 
> Then, I just performed the hot plugout and then plugin operation on
> that drive using qmp.execute's device_del and device_add cmd
> respectively.
> 
> But, after that, the default block size of that drive reverted to 512 bytes.
> 
> admin@node-3:~$ sudo nvme list
> Node                  SN                   Model
>              Namespace Usage                      Format           FW
> Rev
> --------------------- --------------------
> ---------------------------------------- ---------
> -------------------------- ---------------- --------
> /dev/nvme0n1          ashudev-6f34a1cf_13  QEMU NVMe Ctrl
>              1          34.36  GB /  34.36  GB    512   B +  0 B
> 7.1.92
> 
> So, I just wanted to know why the NVMe format type reverted back to
> 512 bytes, as I just performed the hot plugout/plugin operations only.
> Drive's block size (format type) should not be changed upon
> removal/insertion, right ? or am I missing something ?
> 
> Regards,
> Ashutosh
> 

The nvme device (or namespaces) does not have any persistent state. The
only way to specify boot-to-boot configuration is through device
parameters. In other words, if you reformat to 4k, then you need to
change logical_block_size on the device/namespace upon reboot.

The format command mostly exists for testing purposes.

I guess it's a caveat we should be more clear about in the
documentation.

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