[Bug 1641124] Re: Default failback value is badly chosen

[Nish Aravamudan]

Hello and thank you for reporting this bug! Changing default values is a
scary proposition (to me), because we have to think about every possible
environment.

What are the implications of failback=immediate over failback=manual?

Also, the base configuration for multipath "works" for all cases (I
guess unless you are installing during a failover?); if you know your
setup for multipath should use a different setting, you are able to
manage that easily (as you noted) by changing the configuration file.

The concerning part from the manpage is:

Tell multipathd how to manage path group failback.
To select \fIimmediate\fR or a \fIvalue\fR, it's mandatory that the device
has support for a working prioritizer.

We do not know that every device of every Ubuntu Server instance already
using multipath-tools has a "working prioritizer", do we? I'm not sure
what that even is, but I can guess from context in multipath :)

I'm also going to unsubscribe Ubuntu Server, as right now there is not
(in my opinion) anything for the Server Team to change -- multipath
policy is fraught with danger :) Honestly, we take the default from
upstream, because it 'just works'. If you want to see that change, I
would work with the upstream community (maybe 'immediate to manual',
which uses 'immediate' if it is detected that it can be used ('working
prioritizer') or somesuch), but that's outside the scope of the package
in Ubuntu.

** Changed in: multipath-tools (Ubuntu)
   Importance: Undecided => Wishlist

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Title:
  Default failback value is badly chosen

Status in multipath-tools package in Ubuntu:
  New

Bug description:
  As described by:

  https://help.ubuntu.com/lts/serverguide/multipath-setting-up-dm-
  multipath.html

  The default value for failback is set to manual, instead of immediate.
  This effectively breaks the idea of multipath, which allows for
  upgrading E.G SAN A side, then upgrade B side when A is complete.

  With this set to manual, the system effectively halts. With a system
  containing 4 paths to a block device, you'll see this in logs during
  SAN firmware upgrade:

  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-4/target0:0:3/0:0:3:0/block/sde and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda
  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-4/target0:0:3/0:0:3:0/block/sde and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda
  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:03.0/0000:08:00.0/host7/rport-7:0-1/target7:0:0/7:0:0:0/block/sdc and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda
  November 11th 2016, 13:58:05.000	4	kernel	[585264.496735] sd 0:0:1:0: Asymmetric access state changed
  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-scsi\x2d3600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:03.0/0000:08:00.0/host7/rport-7:0-1/target7:0:0/7:0:0:0/block/sdc and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda
  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:03.0/0000:08:00.0/host7/rport-7:0-1/target7:0:0/7:0:0:0/block/sdc and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda
  November 11th 2016, 13:58:05.000	3	systemd	dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device: Dev dev-disk-by\x2did-wwn\x2d0x600a098038303731702b486638665456.device appeared twice with different sysfs paths /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:03.0/0000:08:00.0/host7/rport-7:0-1/target7:0:0/7:0:0:0/block/sdc and /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/0000:03:00.0/0000:04:02.0/0000:07:00.0/host0/rport-0:0-2/target0:0:1/0:0:1:0/block/sda

  
  With the end result that the filesystem is no longer available. If the root partition is on a multipath device (SAN device), then /bin etc. are gone.

  Suggested fix: Set failback to immediate as default value

  This would ensure that what you would expect to happen, actually
  happens - which is when you upgrade SAN firmware, machines continue
  running like nothing happened due to redundant paths

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