What is the SCSI RAID channels?

Question

What is the SCSI RAID channels? I.e. what is the difference between, say, 1-channel RAID controller and 2-channel RAID controller?

Is it for data throughput, number of RAID arrays I can create on controller, number of disks I can plug into controller or what else?

Solution 1

Traditional SCSI connects several drives to a single channel, they all communicate over the same physicaly wiring/cabling. (Newer forms of SCSI like SAS do not do this). You can connect 7 (or more) drives to a single SCSI Channel.

A 1-Channel controller simple has a single channel, and you can connect 7 (or more depending on the exact type of SCSI) drives to that channel. Multiple channels increase the number of drives, and each channel has a limited amount of bandwidth; putting too many high speed drives on the same channel will limit the porential performance.

The RAID controller then takes theese drives and applies RAID (if you configure it), the RAID controller then presents a logical representation of the RAID Array (it shows one RAID Drive, even if there are several physical drives making up the RAID Array).

Solution 2

A SCSI channel is a physical connector on the SCSI controller, where you connect a SCSI cable; multiple drives (or other SCSI devices) can be connected to a single SCSI channel forming a "chain" (usually ending in a SCSI terminator), and all of them will share the same physical connection and bandwidth; each device will be identified on the channel by its own SCSI ID. Usually, the maximum number of devices is 15, plus the controller itself, for a total of 16 possible IDs.

A single SCSI controller can have multiple channels; this way, it can both manage more devices and provide higher performance.

RAID is actually unrelated to channels: it's a way to logically group many physical disks in a single logical volume, providing redundancy and/or higher performance; but you can have a multi-channel SCSI controller without RAID, a single-channel RAID controller, or a multi-channel RAID controller.

Comments

[timurb]

So the number of channels do not stand with the number of arrays I can create on a single controller? Is it possible, for example, to create 2 of RAID1 arrays on 1-channel controller, or to create 3 of RAID1 arrays on 2-channel controller?

[Massimo]

The number of arrays and the type of RAID you can use depend on the controller you're dealing with; there's no general answer. But it's usually unrelated to the number of channels (although multi-channel controllers tend to be higher-end ones, so they also usually have better RAID support).