SOLVED: Start multipath before LVM (Proxmox/Debian/SAN/QLogic HBA). Boot from multipathed LVM root device.

Problem: Proxmox see duplicated disks instead of one. And cannot boot from it (LVM fails).
Even if multipath-tools-boot is installed and initramfs is updated(as well as grub config).
Cause: possibly initramfs scripts order is broken (thus multipath is not ready yet and LVM can see only duplicated disks)
Solution: Join all necessary steps in one quick custom script and rebuild initramfs with it.

1. Create file /etc/initramfs-tools/scripts/local-top/forcelvm with following content

2. #!/bin/sh
   # See man INITRAMFS-TOOLS(8). Section HEADER.
   # https://manpages.debian.org/jessie/initramfs-tools/initramfs-tools.8.en.html
   
   PREREQ=""
   prereqs()
   {
      echo "$PREREQ"
   }
   case $1 in
   prereqs)
      prereqs
      exit 0
      ;;
   esac
   . /scripts/functions
   # Begin real processing below this line
   
   # Load driver for your HBA
   /sbin/modprobe qla2xxx
   # Load device-mapper multipath support
   /sbin/modprobe dm_multipath
   
   # Start multipath
   # WARNING!!! Your /etc/multipath.conf, /etc/multipath/bindings should be configured prior to this!
   
   /sbin/multipath -v3
   /sbin/multipath -ll
   
   # Check how it works. To debug 
   blkid
   sleep 5
   vgchange -ay
   
   if [ -x /sbin/kpartx -a -x /sbin/dmsetup ]; then
        /sbin/dmsetup ls --target multipath --exec "/sbin/kpartx -a -p -part" >/dev/null
   fi
   
   # tmp. Checking again
   #tmp_my_debug=`blkid`
   #echo $tmp_my_debug
   
   # To debug initramfs:
   # You can boot your kernel with "debug" option
   # and then check /run/initramfs/initramfs.debug after boot
   # https://wiki.debian.org/InitramfsDebug 
   
   # N.B. Useful commands when dropped into the initramfs shell:
   # blkid, multipath -v3, multipath -ll, vgchange -ay and "exit" ;)
   

3. 
   Make it executable
   chmod +x /etc/initramfs-tools/scripts/local-top/forcelvm
   
   

4. Rebuild your initramfs:

   sudo update-initramfs -u -k all
   

5. Rebuild your grub cfg:

    sudo update-grub