Re: "btrfs: harden agaist duplicate fsid" spams syslog

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 11/07/2019 03:46, Anand Jain wrote:
> Now the question I am trying to understand, why same device is being
> scanned every 2 mins, even though its already mount-ed. I am guessing
> its toggling the same device paths trying to mount the device-path
> which is not mounted. So autofs's check for the device mount seems to
> be path based.
> 
> Would you please provide your LVM configs and I believe you are using
> dm-mapping too. What are the device paths used in the fstab and in grub.
> And do you see these messages for all devices of
> 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 or just devid 4? Would you please
> provide more logs at least a complete cycle of the repeating logs.

My setup is quite complex, with three btrfs-over-LVM-over-LUKS
filesystems, so I will explain it fully in a separate message in case it
is important. Let me first answer your questions regarding
4d1ba5af-8b89-4cb5-96c6-55d1f028a202, which was the example I used in my
initial log extract.

4d1b...a202 is a filesystem with a main mount point of /mnt/backup2/:

black:~# btrfs fi show /mnt/backup2/
Label: 'snap12tb'  uuid: 4d1ba5af-8b89-4cb5-96c6-55d1f028a202
        Total devices 2 FS bytes used 10.97TiB
        devid    1 size 10.82TiB used 10.82TiB path /dev/sdc3
        devid    4 size 3.62TiB used 199.00GiB path
/dev/mapper/cryptdata4tb--vg-backup

In this particular filesystem, it has two devices: one is a real disk
partition (/dev/sdc3), the other is an LVM logical volume. It has also
had other LVM devices added and removed at various times, but this is
the current setup.

Note: when I added this LV, I used path /dev/mapper/cryptdata4tb--vg-backup.

black:~# lvdisplay /dev/cryptdata4tb-vg/backup
  --- Logical volume ---
  LV Path                /dev/cryptdata4tb-vg/backup
  LV Name                backup
  VG Name                cryptdata4tb-vg
  LV UUID                TZaWfo-goG1-GsNV-GCZL-rpbz-IW0H-gNmXBf
  LV Write Access        read/write
  LV Creation host, time black, 2019-07-10 10:40:28 +0100
  LV Status              available
  # open                 1
  LV Size                3.62 TiB
  Current LE             949089
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:13

The LVM logical volume is exposed as /dev/mapper/cryptdata4tb--vg-backup
which is a symlink (set up by LVM, I believe) to /dev/dm-13.

For the 4d1b...a202 filesystem I currently only see the messages for
devid 4. But I presume that is because devid 1 is a real device, which
only appears in /dev once. I did, for a while, have two LV devices in
this filesystem and, looking at the old logs, I can see that every 2
minutes the swapping between /dev/mapper/whatever and /dev/dm-N was
happening for both LV devids (but not for the physical device devid)

This particular device is not a root device and I do not believe it is
referenced in grub or initramfs. It is mounted in /etc/fstab/:

LABEL=snap12tb  /mnt/backup2    btrfs
defaults,subvolid=0,noatime,nodiratime,compress=lzo,skip_balance,space_cache=v2
       0       3

Note that /dev/disk/by-label/snap12tb is a symlink to the dm-N alias of
the LV device (set up by LVM or udev or something - not by me):

black:~# ls -l /dev/disk/by-label/snap12tb
lrwxrwxrwx 1 root root 11 Jul 11 18:18 /dev/disk/by-label/snap12tb ->
../../dm-13

Here is a log extract of the cycling messages for the 4d1b...a202
filesystem:

Jul 11 18:46:28 black kernel: [116657.825658] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:46:28 black kernel: [116658.048042] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup
Jul 11 18:46:29 black kernel: [116659.157392] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:46:29 black kernel: [116659.337504] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup
Jul 11 18:48:28 black kernel: [116777.727262] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:48:28 black kernel: [116778.019874] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup
Jul 11 18:48:29 black kernel: [116779.157038] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:48:30 black kernel: [116779.364959] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup
Jul 11 18:50:28 black kernel: [116897.705568] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:50:28 black kernel: [116897.911805] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup
Jul 11 18:50:29 black kernel: [116899.053046] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/mapper/cryptdata4tb--vg-backup new:/dev/dm-13
Jul 11 18:50:29 black kernel: [116899.213067] BTRFS info (device sdc3):
device fsid 4d1ba5af-8b89-4cb5-96c6-55d1f028a202 devid 4 moved
old:/dev/dm-13 new:/dev/mapper/cryptdata4tb--vg-backup


I will, later, provide more detailed configuration information,
including the other filesystems and more logs. As that will be very long
I plan to send it directly to you, Anand, rather than to the list.

Graham