Wheezy installer on IBM Power LPAR with disk multipathing via dual VIOS
Hello,
some time ago i was contacted by Philippe Hermes and Thibaud Besson -
both @IBM France - about an installation walkthrough for Debian on IBM
Power LPARs which i wrote earlier. They've noticed, that with the Wheezy
installer things would go sour, once the installation target disks were
backed by a dual VIOS configuration and thus would show up via two paths
in the installer system.
The quick'n'dirty workaround for this would of course be to only present
the target disks via one vSCSI/NPIV/FC path during the installation phase
and add multipathing support later on, once the system is up and running.
As this is a bit inconvenient, i tried mapping the target disks via both
VIOSs, install just on one disk (e.g.: sda) and leave the second path
representation of the same target disk (e.g.: sdb) alone. Unfortunately
this method failed in the installer script - sorry, forgot to write down
the exact place :-( The cause was, that after writing the partition table
on sda, the installer went on and looked at sdb where it failed to find
any valid partition table/label. The command used for this came back with
non-zero RC and this step blocked the further installation tasks.
The third installation method would be to get multipath support working
during the installer phase. Looking into it more deeply, i eventually
was able to figure out the necessary steps and their correct order[1].
Still, the whole procedure turned out to be riddled with manual inter-
vention and thus seems to be rather fragile and error prone.
Now my question is, would this be something to improve the installer on,
or is this too much of an edge case to burden a general purpose installer
image with?
What really surprised me were several issues:
- the need to manually load the multipath module, although the installer
was booted with "disk-detect/multipath/enable=true" and even after
"multipath-modules-3.2.0-4-powerpc64-di: Multipath support" was selected
in the "Load installer components from CD" step.
- the need for manual intervention with regard to the multipath device
names. It seems to me, that several tools used in the installer out-
put shortened device pathnames (e.g.: mpathap1) which are then put
into the target configuration. Later on the running target system has
full device pathnames (e.g.: mpatha-part1), which breaks things very
badly.
- the "Partition disks" dialog keeps showing the individual "sda" and
"sdb" devices even after they have successfully been placed under
multipath control as e.g. "mpatha". I'd think this is likely to be
confusing for the user. Wouldn't something similar to LVMs "multipath_
component_detection = 1" make more sense, since the user already de-
liberately chose multipathing support?
I also did a quick check with the "Debian GNU/Linux testing "Jessie" -
Official Snapshot powerpc NETINST Binary-1 20131214-16:09." installer
image, but there multipath support seems even worse :-( Specifically i
ran into the following issues:
- multipath dependency /lib/powerpc-linux-gnu/libgcc_s.so.1 was missing
in the above installer image.
- the default multipath 'path_selector "service-time 0"' yields:
Dec 15 13:49:57 kernel: [ 1324.267968] device-mapper: table: 254:0: multipath: unknown path selector type
Dec 15 13:49:57 kernel: [ 1324.267973] device-mapper: ioctl: error adding target to table
After a change to 'path_selector "round-robin 0"' in /etc/multipath.conf
this issue was resolved.
- multipath-tools and multipath-tools-boot were not installed into
"/target".
- both (yaboot|grub)-installer seem to fail due to mpath devices being
unknown to OF:
Dec 15 16:43:52 yaboot-installer: mkofboot: /dev/mapper/mpatha1: No such file or directory
I'll have to check this again, since it could be just another full
vs. short device pathname configuration issue.
I'd be happy to open bug reports on the issues mentioned above, but i
first wanted to test the waters and possibly get some feedback about
this installer szenario being too much of an edge case.
Thanks & best regards,
Frank Fegert
[1] http://www.bityard.org/blog/2013/12/16/debian_wheezy_on_ibm_power_multipath
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