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[bug#41143] [PATCH 1/2] mapped-devices: Allow target to be list of strin

From: Mikhail Tsykalov
Subject: [bug#41143] [PATCH 1/2] mapped-devices: Allow target to be list of strings
Date: Sat, 6 Jun 2020 23:16:07 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Icedove/68.8.0


Thanks for the report, guess that's what I get by assuming instead of testing.

I’ve tried the patches, but `guix system reconfigure` fails(?) with

        guix system: error: exception caught while executing 'start' on service 
        error: <>: unbound variable

Can you test with the attached patch, it seems like `cut` macros is undefined while shepherd service is expanded(?). I fixed it by replacing it with a plain lambda. I managed to mount a flash disk with LVM using this patch, so I think it should be working now.

I’m also curious why sources is a list of VG’s as opposed to a list of device
nodes (i.e. /dev/sdX), like the other mappings. Does `pvscan --cache` not work

I always thought that it wasn't possible to mount LVM by PV's, but looking at man, `pvscan --cache -aay` seems to do the thing. Still, I think that using VG's instead of PV's is better, since you must `pvscan` all PV's in a VG in order to activate it, so you'll still need to think in terms of VG's. This also can be a source of errors, for example adding a device to a VG will result in entire VG not being activated until the configuration is changed and the system is reconfigured. Also I think that LVM doesn't put that much importance on individual PV's as it does on VG's as a place where LV's live.

All of this goes out of the window if you use LVM on only one PV, so it may be possible to have a heuristic that will check if source is a device node as opposed to a VG name and perform appropriate actions, but I'm not sure if the resulting complexity is worth it.


Attachment: 0002-mapped-devices-Add-lvm-device-mapping.patch
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