[Bug 1057054] Re: poor performance after upgrade to Precise

[Peter Petrakis]

There is no feature, that has ever existed, that has the capacity to *at runtime*
examine all attached disks, and cross reference their SCSI INQUIRY data to
a table of available device handlers. That table does not exist, if it did, it
would be miserable to maintain.

I checked the udev rules and initramfs scripts from lucid -> precise. We
never loaded dh modules automatically.

The multipath C code has no facility to modprobe or insmod anything.

So the only logical conclusion left is that the module was loaded without
your knowledge, which means your configuration as it was would never survive a reboot.

If that's not true, and you can reproduce that, I would be interested to see it. However,
even if I had the answer, that doesn't completely make up for a complete lack of
vendor participation in qualifying your SAN with our operating [1]. We cannot be
expected to regression test every SAN in creation and rely on users like you (or vendors)
to test and stay engaged. Please contact your vendor expressing support for official Ubuntu
support for your SAN

multipath-tools is supported by the Community, not Canonical, I volunteer to maintain it.
That multipath section in the server guide? I wrote it with the next precise LTS as the deadline,
months of effort. multipath as a whole is light years better than it was in lucid, or ever for
that matter (many helped).

I'm not disagreeing with you that things are missing and there's certainly room for improvement.
You've pointed out several issues, like the dialog box,man page etc, that's all good stuff, please
file a separate bug for each so we can track them.

It's simply a matter of triage and bandwidth, a good multipath bug can soak weeks of time, so
configuration polish like you mentioned falls to the way side.  However, that sort of work
is low hanging fruit, and doesn't require kernel storage engineer with years of experience to
accomplish. Contributions are most certainly welcome.

FYI, there really isn't a hard spec for multipath.conf, it actually functions a lot like
YAML where keywords are globbed, the values integrated and override the defaults.
There's no one place in the code where you can go and discover "this is how config works",
it's scattered everywhere which makes creating regression tests prohibitive if not practically
impossible.

1. The implication is that multipath may have changed so dramatically from 0.4.8 to 0.4.9 that
the scsi_dh_rdac driver may not have been as necessary. There's no way we could have caught
that on code review, testing was required.

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https://bugs.launchpad.net/bugs/1057054

Title:
  poor performance after upgrade to Precise

Status in “multipath-tools” package in Ubuntu:
  Invalid

Bug description:
  I had a Lucid x64 server working with a Dell MD3000i with 4 paths and
  worked as expected.  I added the "prio rdac" line to the conf file,
  then upgraded to Precise, and removed the old mpath_rdac line and
  reboot one more time, just to be sure.  I did this based on a section
  in https://help.ubuntu.com/12.04/serverguide/serverguide.pdf

  as a "sanity check" test, I'm doing 'pv < /dev/mapper/dellsas1 >
  /dev/null' (friendly names enabled).  On Lucid i'd get about 100MB/s
  After upgrading to Precise I get an almost solid 768kB/s.  If I
  instead use the 4 underlying /dev/sd* devices, 2 give errors as
  expected, and 2 run at about 100MB/s as expected so iscsi seems to be
  working correctly and multipath not.

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