BPN-SAS2-826EL1 Backplane question

[ziggygt]

I recently bought a chassis with BPN-SAS2-826EL1 backplane. This system has a X9DRD-7LN4F mother board. It is wired so that both onboard mini-SAS ports are connected from the MB to the backplane. This seems contrary to the BP documentation that says the normal connect is a single mini-SAS in and a daisy chain out to the next expander. Other chassis documentation says the configuration is 8 drives for drives. 4 SSD/NvMe drives. I want to use with a single input and to daisy chain to the next chassis. Is there special FW in this particular backplane. Can it be reflashed to the 12 drive vs 8 and 4 configuration. Appreciate any insight into this configuration?

 

[kapone]

The documentation for that chassis is...wrong. There's NO configuration for that backplane. It's simply a 12 drive backplane.

- If you use two SAS connections to the backplane, you get a "wide" SAS port connection, equivalent roughly to the bandwidth of two SAS connections (8 ports).
- If you use a single SAS connection, well, that is self explanatory.
- The backplane should have a third SAS connection (4 port) which is for "OUT"). That's what you use to daisy chain expanders/backplanes.

Whether you wanna use a single connection or two, it doesn't matter other than bandwidth to the backplane, no refreshing needed/required.

Edit: gah! I forgot, the 2U EL1 backplane doesn't have 3 SAS ports!. If you wanna daisy chain things, you'll have to use a single connection.

 

[ziggygt]

Thanks. Probably some marketing gibberish about the 8+4 SSD/NVMe capability.

I’ll hook this chassis to one MB port and my 2nd 12 drive chassis on the second port and give up that potential BW advantage. Each expander on separate should be better than daisy chained on a single port setup.

This chassis also has a 2x2.5” chassis connected to 6G/s ports. I plan to use this for SSDs. This might give me a place to see the speed of my 10Gbs network. That would only be for temporary storage.

 

[itronin]

I’ll hook this chassis to one MB port and my 2nd 12 drive chassis on the second port and give up that potential BW advantage. Each expander on separate should be better than daisy chained on a single port setup.

SM docs on this topic are poorly written.

Also a SM hybrid backplane with direct or with expander allows up to max number of backplane slot sas/sata drives minus the number of nvme drives (up to 4 for -n4 variant) that are installed. I have only seen hybrid backplane configurations for SM SAS3 backplanes not SAS2.

Consider this analogy:

think about sas expander backplane connections like stacking network switches. typically you see network stacks in a ring topology. This is most clearly demonstrated with three switches. Even with two switches the recommended stack is two cables thus creating a ring.

Back to interconnecting expanders now but remember expanders are functionally similar to switches. Connecting expanders and hbas in a ring gives you protection from a cable failure and protection from a port failure on the hba (assuming at least 2 x 4channel hba connections (sff-8087 or sff-8643) or a port failure on the expander. You also maximize aggregate bandwidth between all of the HBA channels and all of the interconnected disks with this topology.

that's why on SM expander backplanes you will see 3 (or 4 in the case of SAS3) port connectors on backplanes with a single expander.

consider this example using a 2 port x 8 channel HBA, an expander backplane in the host computer and separate disk shelf with expander backplane.

HBA port 1 (channels 1-4) ---cable----> Host Chassis Expander Backplane <----+
|
cable
|
HBA port 2 (channels 5-8) ---cable---->Disk Shelf Expander Backplane <----+

total 3 sas cables.


all 8 channels can see ALL drives whether the drives are connected to the host expander backplane or the disk expander backplane.
Each channel can address *any* drive thus maximizing aggregate throughput to all drives. This topology also ensures access between the HBA and all disks even if an interconnect port failure *or* HBA port failure occurs.

If you only use 2 sas cables and do not interconnect the expanders then each hba port can only see the drives on the connected expander.
If you use 3 sas cables BUT connect the HBA ports only to the first expander *and* then daisy chain from expander to expander a failure of either the expander's daisy chain ports *or* interconnect cable will drop the drives from the disk shelf.

lastly while this may seem obvious it should be stated: Spanning arrays or drive pools across expanders (or disk shelves) should be done with care and caution. an interconnect failure in a poorly designed configuration may drop all the drives connected to the affected expander. That number of drives is almost surely greater than the total number of parity drives which can be lost at a single time and thus the array or pool will likely become unavailable.

 

[ziggygt]

Very interesting. I had planned to skip that third cable between the expanders. I’ll revise that and order another cable. The SM docs describe the mini-sas as in and out on the version 1 and a separate set of in/out for redundancy on the version 2, not as a network. Thanks, Sure glad I asked.

If I understand correctly the additional cable
Chassis Expander Backplane <----+
|
cable
|
Shelf Expander Backplane <----+
it does not matter which port is in or out.

Now the really stupid question. Can a single expander handle connection to two different hosts if the same drives are not attached to both operating systems? I am just curious, not ready to try it. What does the expander care where the commands come from as in a network. OS certainly would lose track but if disk access was segmented, could that work?

 

[kapone]

could that work?

No.

 

[itronin]

Now the really stupid question. Can a single expander handle connection to two different hosts if the same drives are not attached to both operating systems? I am just curious, not ready to try it. What does the expander care where the commands come from as in a network. OS certainly would lose track but if disk access was segmented, could that work?

What you *can* do is use dual port sas drives but your expanders would all need to be dual ported (and possibly use interposers depending on drive). You will be at extreme risk for your data if you make a mistake. The implementation is there for HA storage where your "head unit" if you will is in a HA configuration with another. You can take a look at the iX systems X10 for a nice small simple system that does exactly this. Your host OS's need to support this and you need a coordination mechanism between the two hosts for failover. Not for the weak of heart - like I said: You will be at extreme risk for your data if you make a mistake.

 

[ziggygt]

Thanks I’ll look into this. I am not going to try it. Back in the dark ages I recall SMD disk could dual port. Entrigued by it. But not in my future