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LSI's SAS DAS SAN

SAN in a rack with no network

Internet Security Threat Report 2014

LSI is announcing a 6Gbit/s SAS switch that enables servers in a rack to share direct-attached storage, turning DAS into a rack SAN.

It comes either as a half-width 2U product, the SAS6160, or a full-width 1U product, the SAS6161. The maximum cable length with active copper wire is 25m and so this is for single or dual rack server + storage configurations.

DAS is storage directly attached to one server which cannot be shared. With a SAS switch multiple servers can connect to their part, or zone, of a larger pool of storage shared between the servers, just like a storage area network (SAN). But there is no need for the connecting complexity and expense of a Fibre Channel fabric or iSCSI Ethernet linking the servers to the storage array or arrays in the SAN.

The SAS protocol is used to connect servers and storage arrays via a switch providing a SAS backplane or fabric - LSI does not like the fabric term, saying it is too confusing. LSI has an existing Lynx 3090 SAS switch (pdf) but this only provides limited bandwidth: 9 ports with 4 lanes per port proving 2.4GB/sec full duplex throughput.

SAS here was being used as an UltraSCSI follow-on. Now we have the faster 6Gbit/s SAS standard and LSI's SAS 6000 switch offers 16 ports, each with 4 lanes, providing 24Gbit/s per port and a total of 384Gbit/s bandwidth which can be used to interconnect 1,000 SAS addresses. This level of performance is for department or small/medium business use and is to be compared with 1 or 10Gbit Ethernet and 4 or 8Gbit/s Fibre Channel.

LSI says it can also be considered for use in very large data centres and cloud environments though.

An included utility can modify port configurations, carry out zoning, manage firmware and show the SAS topology. The switches can be cascaded to increase the port count. Two switches can be paired in a high-availability set-up. Management is in-band via the SAS fabric or out-of-band via Ethernet.

The attached storage array or arrays can hold both SAS and SATA disks, offering a performance tier and a capacity tier of storage. They can also hold SAS or SATA Solid state drives (SSDs).

Recently HP has been talking about the relative rise of DAS compared to SAN, and it is this SAS-style DAS it has in mind, DAS with SAN characteristics. HP has already built storage products using a SAS backplane, such as the X9720 scale-out filer, and we can expect more. A possibility would be an integrated ProLiant blade server, ProCurve networking and StorageWorks product using a SAS switched fabric to connect servers and storage.

LSI says it has shipped more than 18 million SAS integrated circuits and that the two new switches will be ready for OEM customers in production quantities in the second half of this year. ®

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