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Imagine I have a number of block devices, that I know are the same device but who have different identificators (it may be a device cut and clued with dmsetup, exported multiple times through iSCSI, etc.). I can guarantee the same volume of them, yet they may have different types (physical device and FC target, for instance). I want to multipath them via multipath util.

man reads, that I can force different policies, but can I force multipath maps themselves? Whatever is the method for detecting path failure, whatever is the path priority policy, it does not depend on device type or their serial/IQN, so I see no conceptual problem in here. However, I do not find anything like that in documentation.

Any ideas?

Well, it was possible to create iSCSI target from SAS-connected device and export it to self to make it part of multipath, but it's more like a hack than force.

Steps are as follows:

1) dmsetup create disk_full --table '0 781422768 linear /dev/sdb 0' - we create dmsetup-ed disk from physical disk with full size

2) echo "50011731011379ec" > /sys/kernel/config/target/core/iblock_0/disk_full/wwn/vpd_unit_serial - we set the serial the same as the physical disk we want to multipath

3) targetcli /iscsi/iqn.2017-12.com.aa.iscsi:srv/tpg1/luns create /backstores/iblock/disk_full - we add our disk to the portal (it was created earlier)

4) iscsiadm -m node -l - we login to both remote machine with iSCSI target and our own

5) service multipathd start - we start multipath daemon if have not already

6) multipath -ll - voilà, here we have out multipathed disks

Note: without dmsetup trick I could never get the same serials: physical disk serial looks like 50011731011379ec even after export to LIO, while dmsetup-ed disk after LIO export gets 600140550011731011379ec000000000 serial, the same as the remote iSCSI target's serial.