[Bug 644489] Re: constantly changes /dev/disk/by-id/{scsi, wwn}-* LUN symlinks with multipathing

[Peter Petrakis]

I can understand that. Attached is a debdiff of the updated package.
I must stress that may not be the final form of the solution. I'm
chatting with Douglas Gilbert (sg io maintainer) now about this.
So far it appears that using an SD device as the target for sg io
can have unintended side effects, like the ones we've been observing.
If the corresponding SG device was used to begin with, this problem
would have never occurred.

See for yourself, mpath_prio_netapp /dev/sg0

In the meanwhile, this patch adjusts the open flag s.t. the "side effect"
no longer occurs. What's annoying is the upstream multipath (0.4.9) is
still using SD devices for this work but they did change all of their
open flags to O_RDONLY. Which leads me to believe they worked around
this side effect too, perhaps unaware of the fact that they were causing
these events to begin with by using sd devices by default.

You can simply build the mpath_prio_netapp straight from
path_priority/pp_netapp/ assuming build deps are met.

** Patch added: "preliminary patch, awaiting upstream feedback"
   https://bugs.launchpad.net/ubuntu/+source/multipath-tools/+bug/644489/+attachment/2174791/+files/adjust-prioritizer-open-flags-to-avoid-sg-io-side-effects.debdiff

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https://bugs.launchpad.net/bugs/644489

Title:
  constantly changes /dev/disk/by-id/{scsi,wwn}-* LUN symlinks with
  multipathing

Status in “multipath-tools” package in Ubuntu:
  Confirmed
Status in “udev” package in Ubuntu:
  New

Bug description:
  Binary package hint: udev

  udevd constantly changes LUN device node symlinks (devices/LUNs, not
  the partition nodes) in /dev/disk/by-id. udevd uses ~15% of CPU and
  system time is using ~50-60%.

  For example:

  [jwm at syslog01.roch.ny:pts/0 /dev/disk/by-id> ls -l wwn-0x60a98000486e5339576f596675735354 wwn-0x60a98000486e5339576f596675744c36 scsi-360a98000486e5339576f596675735354 scsi-360a98000486e5339576f596675744c36; sleep 1; echo '======'; ls -l wwn-0x60a98000486e5339576f596675735354 wwn-0x60a98000486e5339576f596675744c36 scsi-360a98000486e5339576f596675735354 scsi-360a98000486e5339576f596675744c36
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 scsi-360a98000486e5339576f596675735354 -> ../../sde
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 scsi-360a98000486e5339576f596675744c36 -> ../../sdf
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 wwn-0x60a98000486e5339576f596675735354 -> ../../sde
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 wwn-0x60a98000486e5339576f596675744c36 -> ../../sdf
  ======
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 scsi-360a98000486e5339576f596675735354 -> ../../sdg
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 scsi-360a98000486e5339576f596675744c36 -> ../../sdh
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 wwn-0x60a98000486e5339576f596675735354 -> ../../sdg
  lrwxrwxrwx 1 root root 9 2010-09-21 16:12 wwn-0x60a98000486e5339576f596675744c36 -> ../../sdh

  All other device nodes stay the same, such as the device nodes for the
  partitions:

  [jwm at syslog01.roch.ny:pts/0 /dev/disk/by-id> ls -l scsi-360a98000486e5339576f596675735354-part1; sleep 1; echo '======'; ls -l scsi-360a98000486e5339576f596675735354-part1
  lrwxrwxrwx 1 root root 10 2010-09-21 15:47 scsi-360a98000486e5339576f596675735354-part1 -> ../../sdg1
  ======
  lrwxrwxrwx 1 root root 10 2010-09-21 15:47 scsi-360a98000486e5339576f596675735354-part1 -> ../../sdg1

  
  I'm not entirely sure whether this is udev's problem or something related to multipathing. Our most recent experience with multipathing is the last LTS release (hardy), which doesn't exhibit this behavior given similar configurations.

  
  [jwm at syslog01.roch.ny:pts/0 ~> sudo multipath -ll
  rootvol (360a98000486e5339576f596675735354) dm-1 NETAPP  ,LUN           
  [size=36G][features=1 queue_if_no_path][hwhandler=0]
  \_ round-robin 0 [prio=8][active]
   \_ 2:0:2:0 sda 8:0   [active][ready]
   \_ 3:0:2:0 sde 8:64  [active][ready]
  \_ round-robin 0 [prio=2][enabled]
   \_ 3:0:3:0 sdg 8:96  [active][ready]
   \_ 2:0:3:0 sdc 8:32  [active][ready]
  syslog-data (360a98000486e5339576f596675744c36) dm-0 NETAPP  ,LUN           
  [size=1.0T][features=1 queue_if_no_path][hwhandler=0]
  \_ round-robin 0 [prio=8][active]
   \_ 2:0:2:1 sdb 8:16  [active][ready]
   \_ 3:0:2:1 sdf 8:80  [active][ready]
  \_ round-robin 0 [prio=2][enabled]
   \_ 3:0:3:1 sdh 8:112 [active][ready]
   \_ 2:0:3:1 sdd 8:48  [active][ready]
  [jwm at syslog01.roch.ny:pts/0 ~> cat /etc/multipath.conf 
  multipaths {
          multipath {
                  wwid            360a98000486e5339576f596675735354
                  alias           rootvol
          }
          multipath {
                  wwid            360a98000486e5339576f596675744c36
                  alias           syslog-data
          }
  }

  devices {
          device {
                  vendor                  "NETAPP  "
                  product                 "LUN "
                  path_checker            tur
                  path_grouping_policy    group_by_prio
                  prio_callout            "/sbin/mpath_prio_netapp /dev/%n"
                  failback                immediate
                  rr_min_io               128
                  no_path_retry           queue
          }
  }

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