Multipathing FC SAN question - IBM DS4700

[AlexPT]

Hi,

I have an IBM DS4700 SAN which has two controllers in it. I have an IBM x3655 server with a dual ported HBA in it. Everything in plugged into an IBM (brocade) 200E FC switch.

I am just trying to under stand the paths I have. I originally set it up using only one of the FC ports on the HBA. So there was one route to the FC switch. VC showed me two paths:

vmhba1:0:0 and

vmhba1:1:0

As I understand this layout it means HBA1, target0 or 1 and LUN0, right. So the target is the SAN Controllers? I think I have it but would like clarification.

Now I later added another path to the FC switch from the HBA by plugging in a second piece of fiber cable into the second port. I now see 4 paths. My question is, am I seeing 4 paths because of the fact that either of the SAN controllers can provide access to any LUN? I hope that makes sense. If not maybe I can provide a diagram, screen shot.

[Dave_Mishchenko · Accepted solution]

As I understand this layout it means HBA1, target0 or 1 and LUN0, right. So the target is the SAN Controllers? I think I have it but would like clarification.

You have it right. With your initial setup, you had 2 paths to storage because the one port on the HBA could see both storage controllers. The single port is able to see both storage processors because both are attached to the SAN switch and the zoning allows the HBA to see both.

Now I later added another path to the FC switch from the HBA by plugging in a second piece of fiber cable into the second port. I now see 4 paths. My question is, am I seeing 4 paths because of the fact that either of the SAN controllers can provide access to any LUN? I hope that makes sense. If not maybe I can provide a diagram, screen shot.

Right again. Both HBA ports are connected to the switch and both storage processors are connected to the switch, so each HBA port has 2 paths for a total of 4. And in the case you'll have a zone(s) that allows both HBA ports to see both storage processors.

If you were directly connected from the HBA ports to the DS4700 (i.e. no Brocade switch) then you would only have 2 paths with both HBA ports plugged in.

[DigitalVoodoo]

I have the same SAN here, and you are correct in your statements. The target number corresponds to a storage processor in the DS4700, and since each processor can service the LUNs you will see a different path for each possible HBA to SP combination. We have a similar multipath environment, and we see 4 paths as well.

As a side note, make sure you're running the current firmware for all of the DS4700 components, and that you're running in a RAID 1+0 configuration. We've had some issues with ours, and after a lot of hassles and support calls have settled on these items as "must-haves" with the DS4700.

[AlexPT]

Thanks Dave for the quick response,

It just dawned on me just after I posted, that I could change the ownership of a LUN (with a non PROD VM on it) with the IBM management console. Then see what ESX reports in the storage area. That affirms just what you said.

Thanks!

[AlexPT]

Thanks,

Why RAID 1 + 0? I have one RAID 1 array and 1 RAID 5 array (3 LUNS). No issues at present but its not really PROD with only approx 10 VM's. But you're right that's good practice to get the up to date firmware although its also nice not to touch things if they are working! Personally speaking I am not a fan of IBM gear.

[DigitalVoodoo]

The DS4700 install and config guides explicitly stated that RAID 10 was the preferred/optimal configuration for this type of usage, which was confirmed by their SEs during our support calls. We had initially started with a RAID 5 configuration, but ran into huge performance issues whenever we would either create a new LUN or if we lost a drive in the array. Ours is running in production with 160 VMs, so when the disk came to a crawl during the aforementioned operations we really suffered. If you could guarantee that you'll never go into production with a RAID 5 config and with a small number of VMs then you might be OK, but I've learned you can't guarantee anything when it comes to the exponential growth of the virtual infrastructure. YMMV, of course, but I would really recommend using RAID 1+0 from the start. I'm not a huge IBM fan myself, and I know of others running different SANs in RAID 5 with no issues, so it might just be a DS4xxxx thing.

[AlexPT]

Hi, good info, can you point me in the direction of the Redbook and let me know the specific page number?