On 29.09.21 г. 15:44, Anand Jain wrote:

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flap means going up and down. The gist is that btrfs fi show would show
the latest device being scanned, which in the case of multipath device
could be any of the paths.

  But why the problem is only when a device flaps? Or it doesn't matter?

Because when the device re-appears it will be the last device scanned by
btrfs scanning code. It can be reproduced by following steps:

1) Validate 'btrfs fi show' is showing /dev/mapper/xxxx for all fs's
1) Unplug one of the cables from the FC adapter < this can be simulated
by simply doing 'echo 1 > /sys/block/sdd/device/delete' for the given
path device >
2) Wait for the paths/drives associated with the downed port to disappear
3) Check again that 'btrfs fi show' still lists the /dev/mapper entry
4) Reattach the cable to the hba port <this can be simulated by
rescanning the HBA or w/e bus you have: echo "- - -" >
/sys/class/scsi_host/host1/scan >
5) Check that 'btrfs fi show' is now shows /dev/sdX devices for all mpio
fs's

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  Do you mean 'btrfs fi show' shows a device of the multi-path however,
  'btrfs fi show -m' shows the correct multi-path device?

Yes, that's a problem purely in btrfs-progs, as the path devices are
opened exclusively for the purpose of multiapth.

 Ok. All parts of the test case is with an unmounted btrfs, I am
clarified, now.