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

[David Teigland]

On Wed, Apr 12, 2023 at 05:16:41PM +0800, Zhiyong Ye wrote:
> configure mpath on sda alone on the current stock of machines. And I tested
> this in my environment using lvchange --refresh, but it doesn't switch the
> active LV to use mpatha.

It seems to work for me with the latest version of lvm, which may be due
to improved multipath component detection and/or improved handling of
duplicate PVs.  So the current upstream code might work for you also.

# pvs
  PV         VG           Fmt  Attr PSize    PFree   
  /dev/sdf   ff           lvm2 a--  <931.01g <930.76g

# lvs -o+devices 
  LV    VG           Attr       LSize    Origin Devices                      
  lvol1 ff           -wi-ao----  256.00m        /dev/sdf(1)                  

# mount | grep ff
/dev/mapper/ff-lvol1 on /mnt type xfs (rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota)

# multipath -a /dev/sdf
wwid '3690b11c0000438ad0000057150910423' added

# multipath -l
mpatha (3690b11c0000438ad0000057150910423) dm-4 DELL,MD32xxi
size=931G features='2 pg_init_retries 50' hwhandler='1 rdac' wp=rw
`-+- policy='service-time 0' prio=0 status=active
  `- 7:0:0:4 sdf 8:80 active undef running

# pvs
  WARNING: Device mismatch detected for ff/lvol1 which is accessing /dev/sdf instead of /dev/mapper/mpatha.
  PV                 VG           Fmt  Attr PSize    PFree   
  /dev/mapper/mpatha ff           lvm2 a--  <931.01g <930.76g

# lvs ff
  WARNING: Device mismatch detected for ff/lvol1 which is accessing /dev/sdf instead of /dev/mapper/mpatha.
  LV    VG Attr       LSize   
  lvol1 ff -wi-ao---- 256.00m

# dmsetup table ff-lvol1
0 524288 linear 8:80 10240

# lvchange --refresh ff/lvol1

# dmsetup table ff-lvol1
0 524288 linear 253:4 10240

# pvs
  PV                 VG           Fmt  Attr PSize    PFree   
  /dev/mapper/mpatha ff           lvm2 a--  <931.01g <930.76g

# lvs -o+devices ff
  LV    VG Attr       LSize   Devices              
  lvol1 ff -wi-ao---- 256.00m /dev/mapper/mpatha(1)

Dave