tgt-admin in Ubuntu 16.04 doesn't support bs-type aio :

* aptitude install   libaio-dev   libsystemd-dev
* clone https://github.com/fujita/tgt
* edit usr/Makefile : /usr/include/x86_64-linux-gnu/sys/eventfd.h 
* make SD_NOTIFY=1

https://wiki.archlinux.org/index.php/TGT_iSCSI_Target : 

Create a target:
tgtadm --lld iscsi --op new --mode target --tid <unique target ID> -T <unique iSCSI iqn>

Allow only connection from specific initiators:
tgtadm --lld iscsi --op bind --mode target --tid <target ID> --initiator-name <initiator IQN>

or using the initiator IP:
tgtadm --lld iscsi --op bind --mode target --tid <target ID> --initiator-address <initiator IP>

Map a LUN to the target:
tgtadm --lld iscsi --op new --mode logicalunit --tid <target ID> --lun <unique LUN ID> --bstype aio -b /path_to/lun.img

tgt-admin --update ALL
---------------


cat /etc/tgt/conf.d/xcp.conf
--------------
default-driver iscsi

<target iqn.2018-01.com.iscsi:srv.aio6g>
        backing-store /var/iSCSI/6Ga.img
        bs-type aio
        initiator-name iqn.2014-01.com.example:c870fff7
        initiator-name iqn.2014-01.com.example:751f5e43
</target>
				
<target iqn.2018-01.com.iscsi:srv.rdwr6g>
        bs-type rdwr
        backing-store /var/iSCSI/6Gr.img
        initiator-name iqn.2014-01.com.example:c870fff7
        initiator-name iqn.2014-01.com.example:751f5e43
</target>

<target iqn.2018-01.com.iscsi:srv.target0>
        backing-store /dev/mapper/iSCSI_VG-iSCSI_LV
        allow-in-use yes
        initiator-address 192.168.100.10
        initiator-address 192.168.100.11
</target>
------------

Additional info :
https://www.cyberciti.biz/tips/howto-setup-linux-iscsi-target-sanwith-tgt.html

http://scst.sourceforge.net/comparison.html