Multipath JBOD storage devices are not shown via /dev/mapper but each path as a single device.

MAAS does not currently support multipath devices. Curtin, the tool MAAS uses to perform installations, does. We'd have to add support for gathering information on multipath devices during commissioning, a way to properly configure multipath storage, and a way to test multipath devices.

I guess you could either probe for any mapper devices and detect multipath this way or do it over a serial number when collecting info about storage if the same serial more then once then just grab the first one or look for a mapper device.
Do you know where in the code it is so I could have a look myself?

I looked at adding support for multipath on IBM Z. In that case each LPAR has access to mutlipath devices from all other LPARs. There is no way to locally determine which multipath device is for which LPAR, nor is there a way to determine if any are in use. IBM told me that its expected that a storage administrator tells users which multipath device to use. They were adding an API to get that information but it would still be configured manually in the JBOD.

The code for storage is in various places and would require alot of work to add multipath support

* Storage data actually comes from LXD - https://github.com/lxc/lxd/blob/master/lxd/resources/storage.go
* The data is processed in the metadata server - https://git.launchpad.net/maas/tree/src/metadataserver/builtin_scripts/hooks.py
* It has to be modeled. We model block devices and physical block devices. For multipath we'd most likely need to design a new model - https://git.launchpad.net/maas/tree/src/maasserver/models
* The API would have to be updated to interact with the new model - https://git.launchpad.net/maas/tree/src/maasserver/api/blockdevices.py
* The websocket would also have to be updated - https://git.launchpad.net/maas/tree/src/maasserver/websockets/handlers/node.py
* The preseed, which generates Curtin config, would require changes - https://git.launchpad.net/maas/tree/src/maasserver/preseed_storage.py
* We'd also want to update the UI to show that this is a multipath device - https://github.com/canonical-web-and-design/maas-ui

Hi, sorry for the late reply, but the issue is still there. The machine-resource binary is still only listing each disk. I think the correct way of doing it would be first to check multipath, e.g. multipath -ll and if any disks found there you would have to discard them from standard SATA disk output. I can see MAAS is now using LXD resource API and they don't care about multipath so maybe fixing it there would fix it for MAAS at the same time.

Hi, as of MAAS 3.1 and 3.2 Beta the issue is still present. I have several multipath devices and they still show multiple devices instead of an mpath device.

Hi,

This is still a bug in MAAS 3.2.6. This will be a problem for PS6.

Thanks !

Junien, and Mirek, could you both please attach the output of machine-resources and 'multipath -ll'? It would be good to see if both your setups require the same fix.

machine resources : https://pastebin.canonical.com/p/WbQHf2sStJ/
multipath -ll : https://pastebin.canonical.com/p/ngTTs63jSk/

we have the this issue during PS6 deployment , MAAS 3.2.6 list all multipath devices separately, so it is not possible to configure bcache using multipath directly in MAAS

The workaround (credit to Junien Fririck) is to do the bache config in post-deployment, as following:

1. make-bcache -C /dev/nvme6n1p1 -B /dev/mapper/mpatha --writeback
2. copy /lib/udev/rules.d/69-bcache.rules to /etc/udev/rules.d/ folder, modify the rules to not register backing device (eg sd*) , so those device can still be used by multipathd

    cp /lib/udev/rules.d/69-bcache.rules /etc/udev/rules.d/.
    sed -i 's/sr\*/sr\*|sd\*/' /etc/udev/rules.d/69-bcache.rules

    # line 7 of modified rules look like this
    KERNEL=="fd*|sr*|sd*", GOTO="bcache_end

3. normally if bache is configured in MAAS, curtin will create udev rules to link the bache to /dev/disk/by-dname , here we need to do it manually

    uuid=$(bcache-super-show /dev/mapper/mpatha | grep dev.uuid | awk '{print $2}')
    echo "SUBSYSTEM==\"block\", ACTION==\"add|change\", ENV{CACHED_UUID}==\"$uuid\", SYMLINK+=\"disk/by-dname/bcache-osd1\"" > /etc/udev/rules.d/bcache-osd1.rules

4. trigger udev rules

    sudo udevadm trigger

5. update initramfs

    update-initramfs -u -k all

Note that this workaround only works if you don't have sdX devices as part of bcache (in our case, we only have nvme* and mpath* devices). If you have both mpath and sdX devices as part of bcache, then you'd need something more elaborate.

This bug is caused by /lib/udev/rules.d/69-bcache.rules (from bcache-tools) grabbing all block devices where ID_FS_TYPE=bcache, including the disks backing the multipath device. Latter when multipathd daemon comes up and tries to initialize the mpath devices, it fails because those disks are already in use.

Status changed to 'Confirmed' because the bug affects multiple users.