I was playing a bit with multipathed devices (over two FC HBAs on SAN) last week
(I had some hardware for limited time to play with). The support for
multipathing in device mapper seems to be a bit newer thing, and I was left
with couple of questions (things I haven't managed to try out while I had the
hardware)...
Is it possible to have root file system on multipathed device? I guess in this
case some multipathing setup would need to occur when init scritps from initrd
image are run, but I don't think mkinitrd is capable of generating such scripts
(I might create it by hand, though, but than it wouldn't be recreated correctly
on kernel upgrades).
What about LVM? If I define physical volume on multipathed device, when vgscan
is run from initrd image, and say multipathing is not setup yet, it will scan
(for example) /dev/sda and /dev/sdb, which both point to same device. Is it
going to be confused when it finds something that looks to it as two identical
physical volumes?
Or if vgscan is run after multipathing is setup, it would see something that
might look to it as three identical physical volumes. /dev/sda and /dev/sdb
which represent two paths to the same device, and then /dev/mapper/WWPN (where
WWPN is long hex number) which is just multipathed mapping to sda and sdb. Or
would it be smart enough not to use sda and sdb in this case, and use only
"multipathed" device entry from /dev/mapper?
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