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Roundup: Three 16-Port Enterprise SAS Controllers

Roundup: Three 16-Port Enterprise SAS Controllers
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Even entry-level servers come with dual- or quad-core processors and many gigabytes of RAM. But a proper storage subsystem still depends on powerful and flexible host adapters, usually with RAID capabilities. We have three 16-port high-end SAS cards from Adaptec, Areca, and Promise in-house and are ready to run them through their paces in search of a winner.

What SAS is All About

While server storage used to center on adapters and drives employing the Small Computer System Interface (SCSI), today’s interface choice for Direct Attached Storage (DAS) applications is called SAS: Serial Attached SCSI. The parallel SCSI bus had insurmountable issues, such as varying signal run-time on each of the wires at increased speeds, which is why a serial transmission was deployed. SAS is a serial point-to-point connection protocol that doesn’t require signal termination. It works with an 8b/10b encoding scheme. It allows a speed of 3 Gb/s in today’s implementation, with 6 Mb/s coming up this year (representing 300 MB/s and 600 MB/s net throughput per port, respectively). On the surface, that 300 MB/s may not appear faster than the 320 MB/s of UltraSCSI, but the throughput is available per connected device, rather than being shared.

SAS controllers (also known as initiators) utilize SSP, the SAS SCSI Protocol, to talk to client devices (known as targets). The SATA Tunneling Protocol (STP) also lets them utilize Serial ATA drives, and the SAS Management Protocol (SMP) is used to manage expanders. SAS uses both fanout expanders and edge expanders, which can be compared to switches in the networking world. One SAS controller can work with up to two edge expanders, which are in turn used to run up to 128 drives. Fanout expanders allow hooking up even more edge expanders.

The beauty of SAS is that it is extremely flexible and scalable. You can use a variety of configurations within a SAS domain, consisting of SAS and SATA drives set up to provide solutions for various performance and capacity requirements. The cards we looked at are all in the $1,000 range, and provide excellent features and performance for enterprise storage solutions.

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    Anonymous , 24 April 2009 21:58
    what firmware did you test on the areca card? i heard there were some performance issues with earlier firmwares (current is 1.46)
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    jwoollis , 14 May 2009 17:26
    I am disappointed in this review, it does not seem to go into a level of detail sufficient to do justice to this topic.

    Firstly most companies these days would build systems using more than 16 drives or 1x4U rackmounted device, and use iSCSI to distribute storage rather than using disks mounted in each server. It's likely that many companies will have several of these using 32,64,128,256 or more drives and also use this storage in parallel to avoid risk of controller or hardware (other than disks) failure.

    You neglect to mention that each port on the controller card in supporting upto 4 drives with a fanout cable or more drives through the use of edge and fanout connectors runs in Full Duplex that is to say 4x3Gb/s or 12Gb/s in both directions and of course this will double with the next generation 6Gb/s connections. With conventional SATA/SAS drives peaking at between 100 and 200MB/s it is possible for a single port to support much more than four drives, In practice between 6 and 12 drives transfering data continuously in one direction at full drive speed would be required to use up the full bandwidth of one port. If you allow for the fact that drives are rarely use in this manner and not for sustained periods of time and that the drives may be separated into groups rather than used as a single huge RAID array, it would be possible to actually use between 16 and 64 drives off a single port. This of course might be seen as bad practice if a controller has enough ports to separate the drives into smaller groups but the point is that the controllers are far more versitile and this article does little to inform us of this fact.

    You neglect to mention that there is the possibility to acquire Edge and Fanout Expandors as either 3.5" or 5.25" Drive Bay mounted devices or a circuit board which can be installed in both Free Standing Cases or any PC/Server Case to facilitate the use of more drives per port per controller card than would normally be possible with fanout cables.

    There are some who would rather build custom/bespoke solutions rather than pay the extra-ordinarily large sums of money that is required for a 16 bay rackmounted storage solution which are often prohibitively expensive.

    You also neglect to performance test these controller cards by testing performance to the limits of the controller card when used with multiple edge and fanout expanders.

    Please when investigating such topic, do us the service of covering all aspects of the topic properly and in detail so that we might make an informed decision. The controller card is only one part of this solution and the cost of such addons may range from £8,000 to £24,000 per rackmount bay depending on the number of disks supported and the amount/size of disks preinstalled. Perhaps you might offer us examples of these addons with specifications and a Cost per GB! That will certainly put things into perspective!