[CentOS] Multipath and iSCSI Targets

nate centos at linuxpowered.net
Fri Mar 19 16:12:53 UTC 2010


Joseph L. Casale wrote:
> Just started messing with multipath against an iSCSI target with 4 nics.
> What should one expect as behavior when paths start failing? My lab setup
> was copying some data on a mounted block device when I dropped 3 of 4 paths
> and the responsiveness of the server completely tanked for several minutes.
>
> Is that still expected?

Depends on the target and the setup, ideally if you have 4 NICs you
should be using at least two different VLANs, and since you have 4 NICs
(I assume for iSCSI only) you should use jumbo frames.

With my current 3PAR storage arrays and my iSCSI targets each system
has 4 targets but usually 1 NIC, my last company(same kind of storage)
I had 4 targets and 2 dedicated NICs(each on it's own VLAN for routing
purposes and jumbo frames).

In all cases MPIO was configured for round robin, and failed over
in a matter of seconds.

Failing BACK can take some time depending on how long the path was
down for, at least on CentOS 4.x (not sure on 5.x) there was some
hard coded timeouts in the iSCSI system that could delay path
restoration for a  minute or more because there was a somewhat
exponential back off timer for retries, this caused me a big
headache at one point doing a software upgrade on our storage array
which will automatically roll itself back if all of the hosts do
not re-login to the array within ~60 seconds of the controller coming
back online.

If your iSCSI storage system is using active/passive controllers
that may increase fail over and fail back times and complicate
stuff, my arrays are all active-active.

nate




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