Managing RAID on Linux

Cases, Cables, and Connectors

Just because you decide to build a software RAID or use an internal disk controller does not mean you need to fit all your drives into a single server or desktop case. In fact, you can chain as many devices as you want to your Linux system, keeping in mind the limits on devices per channel. Remember that ATA is limited to 2 devices per channel, whereas SCSI is limited to 7 or 15 devices per channel.

By housing drives in external cases and connecting them to the external port’s disk controller, you can create a formidable storage device. Putting disks in different cases will not cause a noticeable performance hit. However, don’t forget that there are maximum cable lengths between devices on individual channels. ATA has a cable length limit of about .5 meters. The cable length limits of an SCSI channel depend on the specific SCSI protocol and transmission type (see Table 2-7).

Table 2-7. SCSI cable length limits

SCSI type	Maximum data throughput (MB/s)	Maximum cable length (meters)
SCSI-1, SCSI, Narrow SCSI	5	6[a]
Fast SCSI, Fast Narrow SCSI	10	3[a]
Fast Wide SCSI	20	3[a]
Ultra SCSI	20	3[a]
Ultra Wide SCSI	40	3[a]
Ultra2 SCSI, Ultra2 Narrow SCSI	40	12[b]
Ultra2 Wide SCSI	80	12[b]
Ultra3 SCSI, Ultra 160 SCSI	160	12[b]
[a] Single-ended [b] Low voltage differential

The cable length limit applies to the total number of devices on a single channel, including external devices. Remember to take into account not only the cable connecting your controller to the external casing, but also the ...