Re: discuss about commit 3b0f9ce: filter-mpath: get wwids from sysfs vpd_pg83

[David Teigland <teigland@redhat.com>]

On Wed, Nov 15, 2023 at 07:35:49AM +0000, Martin Wilck wrote:
> How would that addition of the multipath device come to pass?
> By someone explicitly calling "lvmdevices" on it?

Yes, by running lvmdevices --adddev /dev/mapper/mpatha, or by running
lvmdevices --update, which replaces a componenet device with the multipath
device.

> 1 - someone creates a PV on a normal single-path SCSI device
> 2 - on subsequent boot, a new path has been added (e.g. by a Fibre
> Channel zoning change)
> 3 - what happens before the user makes any change to system.devices?
>     a) LVM attempts to add one of the SCSI (path) devices as PV
>        (which might be grabbed by multipathd already)
>     b) LVM attempts to add the multipath device automatically,
>        somehow figuring out what happened
>     c) neither is attempted, and the PV is missing

If a scsi device suddenly becomes a multipath component, then C.  The
component device is ignored, the PV is missing, and lvm prints:

  WARNING: devices file is missing /dev/mapper/mpatha (253:1) using multipath component /dev/sda.
  See lvmdevices --update for devices file update.

https://sourceware.org/git/?p=lvm2.git;a=blob;f=lib/filters/filter-mpath.c;h=854c26a7c7284c9e566dbd89ea899e554a1c8ed9;hb=HEAD#l37

Maybe there's something better we could do there.
Dave