Serial-Attached SCSI (SAS) technology has been available for a few years. So far, SAS has proven itself to be a robust technology through various product implementations; there have been no major complaints from storage end-users. It seems like SAS-based products have fulfilled the design proposition of the SAS protocol, which is to overcome the I/O bus limitation of the Parallel-SCSI protocol in order to achieve higher performance and scalability.Hence, many customers are now wondering when they will be able to replace their costly Fibre Channel (FC) disks with more affordable SAS disks. For some of them, the answer is today, and for some of them, the answer is indefinite.
Customers with direct-attached external RAID disk systems can migrate from FC-based disk enclosures to SAS-based disk enclosures by swapping the FC HBAs inside the server with SAS host adapters (for JBOD) or SAS RAID cards. FC HBAs typically do not offer RAID capability and often rely on host-based software RAID. SAS JBOD adapters are available for those users who prefer software RAID. However, more users today prefer to deploy hardware RAID (host adapter with RAID-processing ASIC) to avoid the performance impact of RAID processing on the host.
Customers with storage servers, such as Windows- and Solaris-based network storage servers, can enjoy SAS today as well. Storage servers are made from industry-standard servers running storage-serving operating systems. They contain direct-attached storage subsystems, which can be SAS-based enclosures. For example, the Sun Storage 7000 series is one commercial-grade storage server implementation that employs SAS-based disk subsystems. Note that the Sun 7000 platform does not use hardware RAID, but implements the Solaris ZFS zRAID.
Customers with network storage systems, known as SAN platforms, may face a longer, indefinite FC to SAS transition cycle. While all the entry-level network storage platforms from major storage vendors – such as the Dell|EMC AX4, HP MSA2000, HDS SMS100, IBM DS3000, and NetApp FAS2000 systems – have already transitioned to SAS-based disk subsystems, the midrange and high-end storage platforms from these vendors are still using FC-based disk subsystems. The only exception is the midrange HDS AMS2000 line, which provides a large number of I/O slots to populate SAS adapters and connect to SAS-based disk enclosures. To date, the AMS2000 series is the only midrange network storage platform from a major storage vendor that supports FC host connectivity with a SAS back end. None of the high-end platforms have transitioned to a SAS back end.
Vendors have their reasons for not switching to a SAS back end. Midrange and high-end storage platforms from major storage vendors such as EMC, HDS (resold by HP and Sun), and IBM commonly employ a switched-disk subsystem design. A switched design provides non-blocking, point-to-point connection between disks and controllers. Engineers argue that a switched connection is more advanced than a serial connection. Serial connections can potentially become congested and cause performance degradation when they are shared by heavy I/O operations. Hence, it is very likely that the midrange and high-end storage platforms from these vendors will retain their switched back-end architectures for the foreseeable future. However, the HDS AMS2000 midrange arrays producedindustry-leading SPC-1 benchmark results, and did not show any signs of I/O congestion for the test workload.
Nevertheless, SAS is without a doubt more affordable than FC. The traditional FC SAN platforms are increasingly being challenged on the cost front by storage servers based on industry-standard technologies. As more and more customers are attracted to the affordability of SAS-based disk systems, the leading storage vendors may have no choice but to switch to a SAS-based back end. However, it is also possible to implement a SAS switch, instead of an FC switch, to have the best of both worlds. Such a SAS switch is available today – just not in a storage product implementation. Take for example, the HP ProLiant BL SAS switch. Therefore, while the day that SAS disks replace FC disks in high-end network storage platforms is still unknown, it may not be too far away.
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