[linux-lvm] Removing VG mappings using dmsetup tool

[Zdenek Kabelac]

Dne 23. 06. 20 v 16:26 Vojtech Juranek napsal(a):
> Hi,
> I'm working on storage part of oVirt project [1]. Besides other options, we
> provide iSCSI storage for VMs. We create PV(s) on attached LUN(s), create
> volume group from this PV(s) and then create LVs on this VG as needed (each
> disk on its LV, similar there's dedicated LV for our metadata etc.). VG is
> what we call (block) storage domain.
> 
> When we remove storage domain we deactivate all LVs and remove LVs and VG from
> all hosts. However, when this fails (e.g. storage is for some reason
> unavailable), hosts are left with stale LVs and VG which can cause various
> issues for us. What we are going to do is to remove these stale VG mappings
> using dmsetup remove command (first we try to remove LVs using lvm, but if
> there are still some mappings, we will try to remove it using dmsetup).
> 
> I'd like to head your opinion on this approach. Is it wise to use dmsetup in
> such case? Or is there any better way how to handle such situations or remove
> stale mappings?

Hi

Of course you can remove 'stale' mapping with 'dmsetup remove' -
(after all lvm2 is doing basically very similar job like
'dmsetup create/remove/load/suspend/resume'....

BUT I'd probably focus rather on fixing your oVirt workflow - you should
not be 'losing' devices while you have activate mappings on them - that's
IMHO major desing flaw - you can very easily cause very SERIOUS damages
to your on disk data consistency - so disk would require serious fsck before 
reuse...

Note - you cannot 'remove' mappings 'in-use'  (aka open count of a device
is higher then 0  - see 'dmsetup info -c' output for this).
However you can replace such mapping with 'error' target - so the
underlaying device is relaxed - although we do not support this
in lvm2 - you would need to use 'dmsetup' for this (and open lvm2 RFE
if there would be some serious justifaction).

Regards

Zdenek