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

[Zhiyong Ye]

Hi Dave,

Thank you for your reply.

Because I had not considered multi-pathing before, there is no way to 
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.

# lvchange --refresh is-he57e6mr4wemb19kgf8i
WARNING: Not using device /dev/mapper/mpatha 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.

# pvs
WARNING: Not using device /dev/mapper/mpatha 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

This is because duplicate PVs are encountered during the disk discovery 
phase, but lvmcache tends to use sda because it is currently being used 
by LV and mpatha will be dropped.

I debugged the _choose_duplicates function in lvmcache and printed the 
following log:
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25 compare duplicates: 
/dev/mapper/mpatha 254:3. /dev/sda 8:16. device_hint /dev/sda.
vm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: size 21474836480. 
/dev/mapper/mpatha is 21474836480. /dev/sda is 21474836480.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: /dev/mapper/mpatha 
was prev <none>. /dev/sda was prev <none>.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: /dev/mapper/mpatha is 
in subsystem. /dev/sda is not in subsystem.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: /dev/mapper/mpatha is 
dm. /dev/sda is not dm.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: /dev/mapper/mpatha 
has no mounted fs. /dev/sda has no mounted fs.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25: /dev/mapper/mpatha is 
not used for LV. /dev/sda is used for LV.
lvm[3352450]: PV HgL3xRqTiom75QnGAOUn0U8uHkwHbH25 with duplicates will 
change from /dev/mapper/mpatha to /dev/sda.
lvm[3352450]: Dropping unchosen duplicate /dev/mapper/mpatha
lvm[3352450]: Adding chosen duplicate /dev/sda

Is there any way to change the PV preference policy in _choose_duplicates?

Regards

Zhiyong

On 4/11/23 9:31 PM, David Teigland wrote:
> On Tue, Apr 11, 2023 at 01:29:42PM +0800, Zhiyong Ye wrote:
>> 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
> 
>> May you please advise how one can convert the pv used by vg from sda to
>> matha without affecting the current lv usage.
> 
> lvchange --refresh should refresh the active LV to use mpatha.  To avoid
> this, you might configure mpath on sda alone prior to adding the second
> path.
> 
> Dave
>