Hello,
On Mon, 2021-12-20 at 13:29 +0300, Vadim Akimov wrote:

From my limited experience, it would be better installing at some
(extra) HDD in "normal" mode and then copying everything to newly
formatted btrfs volume with all options as you like. After that, you
do usual 'chroot, grub-install' thing et voila.

Is that experience from before Bullseye?  I heard that the Bullseye
installer has better Btrfs support.

Also from my experience, it's better not to use btrfs for qemu 
images at all.
[...]
Even with such file you'll get synchronous writes in the VM 3-4 times
slower than you'd have with image on ext4.

Does Btrfs unsuitability to QEMU VM images relate exclusively to
synchronous write *performance*, or does it also harm SSD lifetime
(assuming nodatacow and raw format)?  I intend to give my VM a second
disk image.  One image will be on the SSD (holding system files) and the
other on the SATA HDD (holding user files).  The NVMe SSD is probably
fast enough that the VM will have overall good performance even with the
synchronous write slowness you mentioned; but would it excessively wear
the SSD?  Do I have to create an ext4 partition on the SSD just for the
QEMU VM disk image?

you still can have swap in a file over btrfs, no need for separate
partition.

I am aware Btrfs has official support for swap files (with some
restrictions), but is it reliable, efficient and light on the SSD
lifetime?  The Debian wiki recommends against swap file on Btrfs
(although some parts of Debian wiki are visibly outdated).

Thank you for your help!

Kindest regards,
  Jorge