Q

Fibre Channel/SCSI performance gains

Do Fibre Channel drives offer any performance gains over SCSI drives? I know that dual-porting increases reliability.

Newer Fibre drives usually contain improvements in both mechanics (seek time, rotational latency) and logic (more on drive cache, improved electronics) and offer other benefits if used in a SAN. In SAN environments, the interconnect is serial Fibre instead of parallel SCSI. Therefore, the drive electronics do not have to implement a SERDES chip (serializer, deserializer) in the data path of the drive electronics thus, adding a bit of latency to I/O. This provides a slight performance benefit for Fibre drives in a SAN. Drive performance is also relative to the environment where used. By this I mean if the storage enclosure, the drive is fitted into supports dual ported drives, but the controller only issues commands one port at a time (second port for failover only), there is no performance gain from using dual ported drives. Some storage array vendors (HDS is one) queue I/O to both ports of the drive (active/active), and therefore gain a performance edge. Another method is to create RAID sets where dual ported Fibre drives are used in a "QUAD LOOP" environment (4 FC-AL loops) internally, where odd drives in the raid set are on one dual ported path, and even drives on another. This essentially provides four ports to queue I/O on in a single raid set.

Finally, there is the benefit of pure Fibre itself. In environments where bandwidth is the primary concern (video serving for instance) the 100M bytes per second pipe Fibre provides on EACH PATH of a dual ported drive surpasses even Ultra3 SCSIi in performance. And, the Fibre spec only gets faster in the near future (2G bytes then 10G byets!)


This was first published in March 2001

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