SAN/Networking/Linux: Using Multipath To Verify and Troubleshoot Connectivity To FC LUNs

A few days ago there was in error on one of our Cisco MDS 9120 fiber switches. The current environment at this datacenter consists of two Cisco MDS 9120 SAN switches with servers redundantly connected between the two. These switches are used to connect the servers to our fiber channel (FC) storage systems. In this case, the servers are generally mapped to an EMC, NetApp, and two RamSans. Below outlines a basic example of what can be expected from multipathing and the Linux environment when there is loss of connectivity to one leg of the fiber network.

Below is an excerpt of the log coming from a dual fiber connected server. In this case, the max active paths would be 2. If the server had quad fiber connectivity it could have a max of 4 paths in our environment. As seen from the log, multipathd reported the loss of a path to each LUN an logged it “remaining active paths: 1″. Once the errors were corrected in the failed switch, multipathd reported the restoration “remaining active paths: 2“.

[root@testsrv01 ~]# cat /var/log/messages | grep multipathd
May  6 10:30:55 testsrv01 multipathd: TempLUN1: remaining active paths: 1
May  6 10:30:55 testsrv01 multipathd: 8:224: readsector0 checker reports path is up
May  6 10:30:55 testsrv01 multipathd: 8:224: reinstated
May  6 10:30:55 testsrv01 multipathd: RanSanLUN2: remaining active paths: 1
May  6 10:30:56 testsrv01 multipathd: 65:128: readsector0 checker reports path is up
May  6 10:30:56 testsrv01 multipathd: 65:128: reinstated
May  6 10:30:57 testsrv01 multipathd: TempLUN1: remaining active paths: 2
May  6 10:30:58 testsrv01 multipathd: 65:224: readsector0 checker reports path is up
May  6 10:30:58 testsrv01 multipathd: 65:224: reinstated
May  6 10:30:58 testsrv01 multipathd: RanSanLUN2: remaining active paths: 2

To be safe, the ‘multipath’ command was used via the Command Line Interface (CLI) on the server(s) to verify that the storage systems were visable to the OS.

NAME
       multipath - Device mapper target autoconfig
SYNOPSIS
       multipath [options] [device]

       -l     show the current multipath topology from information fetched in sysfs and the device mapper
       -ll    show the current multipath topology from all available information (sysfs, the device mapper ...)

Truncated results are below that shows connectivity to the RamSans and EMC. This server is not mapped to the NetApp.

[root@testsrv01 ~]# multipath -ll
ramsan_DramCache (25050c24001094501)
[size=9 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=4][active]
 \_ 2:0:3:0  sdah 66:16  [active][ready]
 \_ 2:0:4:0  sdai 66:32  [active][ready]
 \_ 1:0:3:0  sdal 66:80  [active][ready]
 \_ 1:0:4:0  sdan 66:112 [active][ready]

ramsan_FlashSSD (25050c24021094501)
[size=1853 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=4][active]
 \_ 2:0:3:1  sdaj 66:48  [active][ready]
 \_ 2:0:4:1  sdak 66:64  [active][ready]
 \_ 1:0:3:1  sdam 66:96  [active][ready]
 \_ 1:0:4:1  sdao 66:128 [active][ready]

EMC_LUN1 (461a963004637699a05bs3464f76)
[size=100 GB][features="1 queue_if_no_path"][hwhandler="0"]
\_ round-robin 0 [prio=2][active]
 \_ 2:0:2:8  sdab 65:176 [active][ready]
 \_ 1:0:2:8  sdl  8:176  [active][ready]

Notes: Make sure to verify that all servers have multipathing setup and working correctly, all of the Linux servers never missed a beat! The database (Oracle) and Websphere servers kept on going. The same can not be said about the Exchange environment. Due to a configuration problem there, multipathing was not functioning and it lost connectivity to its storage LUNs.  The VMware cluster survived fine as well.

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