Mixing FC speeds in a SAN

First, I'm not a SAN guy, but I have been asked to trouble shoot some performance problems we are having in our SAN enviornment.

What we have is:

Hosts_Group01 < -- 2 Gbps -- > |--------------|
                                                   | FC Switch |< -- 4 Gbps -- > SAN Controller < -- 2 Gbps -- > 4 Gbps capable FC/SATA Disks
Hosts_Group02 < -- 4 Gbps -- > |--------------|

Hosts_Group01 have servers that only have 2 Gbps capability, hosts in Hosts_Group02 have and are connected at 4 Gbps.
The SAN controller is connected to the FC switch at 4 Gbps.

The SAN controller has the capability to connect to the Disks at 4 Gbps, but at the moment it is configured to run only at 2 Gbps.

This was done because it is believed that the SAN Controller to disks must run at or below the slow host speed.  I was told that if we change so that the SAN controller to Disks run at 4 Gbps we could get data corruption.

Now my background is networking and this makes no sense to me in a switched environment.

I'm I crazy or should we be able to change the SAN controller to disk to 4 Gbps?

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giltjrAsked:
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DavidPresidentCommented:
First, if your VENDOR, i.e., the manufacturer of the array told you to run the back-end disks at 4Gbit, then better give them the benefit of the doubt.  Not all hardware is qualified to run at 4Gb to begin with, and a noisy, error-prone link can not only cause data loss but performance hits due to LIPs and retries.

Secondly, not all disks can support a 4Gbit interface, so if you have any disks that won't, it will drop to 2Gbit/sec anyway.

SATA disks are simply not capable of running at 4Gbit anyway.  They are obviously using a FC->SATA bridge chip, and no SATA drive on the planet can run at 4Gbits anyway.

So this is all moot.  I don't see how you could possibly get 4Gbit/sec on the disks due to the SATA-attached devices.
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giltjrAuthor Commented:
Our SAN is an IBM DS4700 with multiple EXP810 expansion shelves.  One EXP810 is SATA, which you can run at 4 Gbps and IBM recommends that you run it at 4 Gbps if at all possible.  All of the other EXP810s and the DS4700 have 4 Gbps FC disks installed.

I will call the host to DS4700 the "front" and the DS4700 to the EXP810s the "back".

All of our hardware in the DS4700 SAN is capabel of running at 4 Gbps.

However, our SAN guy is saying that because we have some hosts on the "front" that can only do 2 Gbps that the "back" has to run at  2 Gbps.  Which makes no sense to me.

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DavidPresidentCommented:
Your SAN guy needs to go back to school.  The SAN has no idea what speed the back-end drives are running.  Only the RAID controller knows about those disks, and only the RAID controller cares.  Yes , some RAID controllers may have problems with different front-and and back-end speeds.

But IBM said 4Gb is OK, so they know more about what their hardware will do than your SAN guy, so take their advice :)
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giltjrAuthor Commented:
Thanks.  That is what I thought.  Now to convince our SAN guy.
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DavidPresidentCommented:
Throw him a bone, tell him maybe he wasn't aware it was RAID. and thought it was JBOD.
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giltjrAuthor Commented:
Unfortunately he  knew it was RAID, he setup the whole thing from scratch.

I think he got confused as the back is FC-AL, and the front is FC Switched.  At one time he told me that because we were running in FC-AL everything had to run at the slowest speed, which I do agree with.

However, we are not running FC-AL on the front.  Which I don't even think you can do FC-AL on the front of a DS4700, I think its only options are switched or having the hosts directly attached to it.

The group that handles our distributed servers and SAN's are not really happy with me right now.  They have been trouble shooting a performance problem a few months.  I was asked to "help" them and in 3 days I identified 2 issues.  

This being one of them and they all swear that I'm wrong and that FC protocols are different from networking protocols and so you can't mix slow "front" speeds with fast "back" speeds.  Of course they seem perfectly happy to mix fast front speeds and slow back speeds.

Our higher ups are not really happy with them right now.
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