[linux-lvm] How to convert a single path to a multipath after using dm-multipath

[Zhiyong Ye]

Dear all,

Dm-multipath (multipath-tools in the user-space) provides powerful 
multipathing capabilities to aggregate multiple paths from the same 
storage backend into a single disk device. However, it is difficult to 
extend single paths into multipaths when LVM is already in use.

I used two physical machines to set up the POC environment for testing, 
with one physical machine acting as the storage iscsi multipath server, 
setting up the same LUN to have two paths to access. Another physical 
machine acts as the iscsi client, first using a single path (/dev/sda) 
and creating vg and lv on it. I then turn on the multipath function and 
merge the two paths into a new device (/dev/mapper/matha), and I get the 
following error when I execute the lvm command.

# pvs
WARNING: Not using device /dev/mapper/matha for PV 
HgL3xR-qTio-m75Q-nGAO-Un0U-8uHk-wHbH25.
WARNING: PV HgL3xR-qTio-m75Q-nGAO-Un0U-8uHk-wHbH25 prefers device 
/dev/sda because device is used by LV.
PV           VG                      Fmt  Attr PSize  PFree
/dev/sda     is-he57e6mr4wemb19kgf8i lvm2 a--  10.00t 9.98t


To be able to use the multipathing capability, I tried to convert the 
device from pv to matha using vgimportclone, but it failed since both 
sda and matha were pointing to the same iscsi storage backend.

May you please advise how one can convert the pv used by vg from sda to 
matha without affecting the current lv usage.

Regards

Zhiyong