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Invest in the Future with SAS, SATA and SFF

by Hu Yoshida on Nov 4, 2010

The enterprise storage market is quickly moving toward Serial Attached SCSI, SAS, Small Form Factor, SFF, 2.5 inch drives, and large capacity SATA disks. Both our AMS 2000 modular storage arrays and our recently announced Virtual Storage Platform (VSP) support SAS, SSF, and SATA drives. IBM quickly followed our lead with the announcement of SAS and SFF in their DS8800 disk array.

SAS Advantages
The advantage of SAS drives over Fibre Channel (FC) drives is point-to-point connectivity over FC Arbitrated Loop. In a FC Arbitrated Loop, a drive must arbitrate to get access to the loop. This has a performance impact, especially if some of the drives are much faster than the others, as in the case of SSD versus Hard Disk Drives (HDD). It also has an availability impact when a disk fails since the maintenance program must query each disk in the loop until it finds the bad disk. With point-to-point SAS, these problems go away. Point-to-point also enables the use of in-band controls and eliminates the cost of out-of-band controls. The latest SAS connections are also 6 Gbps versus current 4 Gbps FC loops. SAS is also compatible with SATA. SAS is dual ported while SATA is single ported, so a SATA drive can be connected and use only one of the ports on a SAS connector.

SAS is an ideal connectivity protocol for Hitachi Dynamic Tiered storage where a volume can be formatted into pages, and the pages are assigned to tiers based on the activity of the page. In this case, a small amount of SAS Flash drives can be used as Tier 0, combined with some SAS HDD for Tier 1, and a whole lot of SATA disks for Tier 2. (see David Merrill’s blog for a comparison of the cost benefits of tiered storage using SSD/HDD/SATA versus a single tier of HDD)
Since SAS is point-to-point, the Flash drives will not drown out the HDD or SATA disks which are also point to point. This same configuration on FC loops will experience contention.

The Benefits of SFF
SFF or Small Form Factor drives are 2.5 inch drives which will have a major impact on environmental savings. A 2.5 inch disk drives consumes about half the power of a 3.5 inch drive and occupies about two-thirds the space of a 3.5 inch drive which have been the norm up to now with enterprise storage. 2.5 inch drives are already widely accepted in low-end systems, internal systems, and consumer applications. This means that the volumes will be there to drive down the costs much faster than the shrinking volumes of 3.5 inch disk drives, especially 3.5 in FC disk drives.


While other storage vendors like IBM are introducing SFF disks, Hitachi has gone one step further by changing the way we package the drives. On the Virtual Storage Platform, we can package 128 SFF drives in a 13 U by 19 inch module. The width of our cabinet is 24 inches. The drives can be serviced from the front or the back by opening the right or left door of fans that are located on the front and back. When we open the front door of fans, the back door of fans increases their speed to keep the temperature in a safe range. Other vendors mount their drives across the front of a drawer with fans in the back. IBM mounts 24 drives across the front and their cabinets are 33.2 inches wide.

A configuration with 1000 drives would take three of our VSP cabinets and a total width of 6 feet. An IBM DS8800 could also provide 1000 disks in three cabinets, but would take 8.3 feet. However, compared to VMax, EMC would require one control cabinet and five disk cabinets, or a width of 15 feet to provide 1000 disks. Since their disks are 3.5 inch FC disks, they would also require almost twice the power.


Here is a three-year TCO comparison that was produced by Wikibon, showing VMAX, DS 8700, DS8800, USP-V, and VSP. Although this seems to indicate that the difference in cost savings is due to FC versus SAS. The major difference is due to SFF drives. The cost impact of SAS will become more evident as the volumes of SAS drives increases over FC drives. The drives business is all about volumes.

If you are investing in a storage system today, look at which will give you the best price performance over the next 3 to 5 years; you should consider storage systems with SAS, SFF, and SATA.

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