Managing RAID on Linux - Derek Vadala [31]
During the life of your array, it's possible that even if you have taken great pains to make sure that all your disks are matched, you may be forced to introduce a disk that is slightly different. For example, what happens if a disk fails and your vendor no longer makes the drives with which you built the array? In that case, you might have no choice but to use a different drive because the cost of upgrading all the disks might be too high. Keeping spare disks on hand in anticipation of a failure is advisable whenever financially possible.
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 internal ribbon cable found inside the external case. In the rare situation that you are working with HVD SCSI, remember that it has a maximum cable length of 25 meters, regardless of the SCSI implementation it uses.
Cables come in two types: cheap and expensive. I strongly recommend that you spare no expense when purchasing cables. I've seen countless system administrators drive themselves insane diagnosing an SCSI performance problem only to later realize that they've bought poor quality cables that could not handle the data load. This mantra applies when using both internal and external cabling. Controller card manufacturers often bundle an internal ribbon cable with new controller cards (unless you buy an OEM version). Use these cables at your own risk; their quality varies greatly between manufacturers. It's probably best to find a good source of reliable cables and use them in all your systems, even when cables come bundled with cases or controllers.
Finding the correct external drive cases can be difficult, especially when working with the latest SCSI protocols. Make sure that the connectors match your card, or you will have to buy an expensive converter cable that can hinder performance. It's also important to make sure that the case is rated for the protocol you are using. Some cases may come equipped with the proper external connectors, but the internal cable might be rated for an older SCSI implementation.
Drives come in two sizes: 3.5″ and 5.25″. The 5.25″ drives can only be placed in 5.25″ bays. These drive bays are usually external, meaning that a plastic piece on the front of the case can be removed to expose the drive. 5.25″ bays are full-height (3.25″).