Database Reference
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path_checker emc_clariion
features "1 queue_if_no_path"
hardware_handler "1 emc"
prio emc
failback immediate
rr_weight uniform
no_path_retry 60
rr_min_io 1000
rr_min_io_rq 1
}
}
Again, this is highly device-specific and should come from the storage vendor.
The last section, multipaths-{} contains specific mapping instructions for individual LUNs. This becomes clearer
with an example:
multipaths {
multipath {
wwid "1IET 00010001"
alias OCR001
}
multipath {
...
}
...
}
This section is optional; multipath devices will be created even if there are no instructions for mappings
between WWID and device name. In the above example, the device with WWID "1IET 00010001" will be configured
as
/dev/mapper/OCR001
. The benefit of adding more human-friendly names is that troubleshooting becomes a lot
easier. Instead of having to hunt down the planned purpose for device
/dev/mpatha
you immediately know why the
device has been created. On the other hand there is added overhead involved in maintaining the mapping. Since
the device naming can be a little bit confusing, here is a summary of how device names are created when using
dm-multipath:
•
If
user_friendly_names
is set to yes, the device will be created in
/dev/mapper/mpath*
.
•
If
user_friendly_names
is set to no, the device will be created as
/dev/mapper/
WWID
which is
very unreadable for humans, making it difficult to find a specific device.
•
If a direct mapping exists in the
multipaths {}
section, the alias name will be used.
•
The internal devices
(/dev/mpath*
and
/dev/dm-*
) are always created, but should not be used
in the context of the Oracle database.
For those who know the device mapper, you might wonder why there is no mention of the gid, uid, and mode
settings. These were very useful in Oracle Linux 5.x to set ownership of LUNs when using ASM. Unfortunately this
functionality has been deprecated, and once more UDEV rules have to be used instead. A recent alternative is the use
of ASMLib. The use of udev and dm-multipath is a little complicated for an Oracle DBA, partly because LVM and the
multipath driver share a very similar interface. Thankfully, the device mapper multipath module takes care of half the
work. The rest needs to be done in a rules file.
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