Re: "relaxed" `-hda file.qcow2` equivalent ?

[Frantisek Rysanek]

> 
> hi,
> what is the "relaxed" equivalent of the `-hda file.qcow2` command line
> argument to control file locking (`locking=off`) ? the documentation
> says that `-hda a` can be replaced by `-drive file=a` and that locking
> can be controlled with the `locking` option of the `blockdev` argument
> : how to link `drive` and `blockdev` to obtain "relaxed" `-hda
> file.qcow2` ? regards, lacsaP.

Let me give you an example of how disk controller device and disk 
drive devices are nested, i.e. the expanded form of the "-hda" arg.
I start qemu by a script, and I use shell variables to put the long 
command line together. The following is a relevant excerpt of the 
script.
See how the "-device" (controller) record maps to the "-drive" 
records via the id= and drive= attributes:


DISK1="-drive 
if=none,id=ff,cache.direct=on,aio=native,discard=unmap,detect-zeroes=u
nmap,file=/somepath/system.qcow2"

DISK2="-drive 
if=none,id=fe,cache.direct=on,aio=native,discard=unmap,detect-zeroes=u
nmap,file=/somepath/data.qcow2"

DISK3="-drive 
if=none,id=fd,cache.direct=on,aio=native,discard=unmap,detect-zeroes=u
nmap,file=/somepath/swap.qcow2"

HDD_STUFF="$DISK1 $DISK2 $DISK3 -device virtio-scsi-pci -device 
scsi-hd,drive=ff -device scsi-hd,drive=fe -device scsi-hd,drive=fd"

In my script, those are four long lines.
Here they have wrapped... sorry :-)

I run this on top of a filesystem that supports sparse files and 
snapshots, and can be told to avoid FS-level COW if it can
 (= to make it prefer in-place overwrites over COW).

Now... I've found a more advanced description of image file locking 
in the official current documentation here:

https://www.qemu.org/docs/master/system/qemu-block-drivers.html#disk-i
mage-file-locking

...now try to combine this with my example above :-)

Not sure what you mean by "relaxed".
Do you need to map the same image file to multiple guests?
For what purpose?
HA failover?
Both/all guests should have simultaneous write access?
Similar to multipath SCSI ? Are you playing with the GFS2/OCFS2 or 
something?
Or do you mean to use the same image by multiple guests as a shared 
read-only disk?
I recall reading an article ages ago, on how to create "ephemeral 
fake writeable block devices" for individual guests, on top of a 
shared read-only "master image"... (as multiple COW snapshots)

Frank