SAS for Blades

Blade-based computing form factors offer many potential advantages for constructing storage subsystems using standard hardware, but until recently, the lack of cost-effective storage connectivity has hampered blades when compared to 1 and 2U servers with directly connected SCSI or SATA/SAS shelves.

As I mentioned in an earlier post about IBM's BladeCenter S storage attachment capabilities, this is starting to change as low-cost SAS storage connectivity switching is integrated into blade solutions.

And now I am pleased to see that HP has introduced a new product, the HP StorageWorks 3Gb SAS BL Switch. This product provides eight external facing SAS 3GB ports, and if you install two of these switches in a c3000 chassis, you can attach up to 16 external SAS/SATA shelves. Assuming 12 TB of SATA storage per shelf, that allows you to have 192 TB of raw addressable storage per c3000.

This is perfect for high density storage systems, as you end up with 6U for the c3000, 32U for the storage shelves, and 4U remaining for a KVM and networking equipment. As the c3000 series blades also allows you to install "Storage Blades", you can host databases locally to the blade. This makes an excellent platform for a scalable high-density StorageGRID deployment, as each site can start with two blades providing basic services, and as storage capacity is expanded, additional shelves and blades can be added as needed. And building out to 200TB per rack is a pretty respectable density. One could easily use this hardware to build a 1 PB storage system based around five racks running StorageGRID, connected together using 10 GB network uplinks.

When comparing this to the IBM offering, there are both advantages and disadvantages. In order to allow an HP blade to access the SAS switch, you have to install a Smart Array P700m controller card, which represents an additional cost. With the IBM offering, the RAID controller in part of the SAS switch module itself. But given that the HP switch allows twice the SAS attachment density, and that having the controller card as part of the blade provides a greater degree of failure isolation, I'm inclined to prefer the HP solution.

But regardless of the minor differences, the bottom line is that both systems are now far more viable for use as lower-cost storage infrastructure due to the elimination of the need for fibre-channel based connectivity to the storage shelves. This is a huge improvement in terms of costs, and one that we won't see again until FCOE (or SAS over Ethernet) becomes widely deployed.

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