Finer Points On RAID Controllers
64 Bit DMA
A controller must be able to address the entire storage area in order to access also in future the main memory content via DMA (Direct Memory Access). That presents no obstacles with a 32 bit system, but it certainly does with 64 bitters such as the Intel Itanium 2 or AMD's Athlon 64. According to the company, RAIDCore has allowed for this scenario - lacking a suitably equipped system we could not corroborate this claim. Naturally, in this regard anything RAIDCore can do Adaptec and LSI do just as well.
Several Arrays Per Drive
A much talked about but rare feature. Although Adaptec promotes it, we had some problems with it. RAIDCore also includes this functionality in its range. It boils down to this: Especially when several hard drives are used, the best suited configuration for each application instance should be possible. Should, for example, with a total of 8 hard drives performance be priority and only a small amount of data are stored in a redundant array system, a large RAID 0 that does not use the entire capacity would be the thing. The rest could be handled by a RAID 5 that stores the really important data. In the case of failure the content of the RAID 0 may be lost, but RAID 5 does the job according to the tried-and-tested procedure.
Interface: PCI Or PCI-X With 64 Bits
Not only should a RAID controller work fast, it should also be able to pass on high data rates to the system. The usual scenario hitherto was 64 bit wide PCI buses with 33 or 66 MHz (3Ware 8506, Adaptec 2200S, LSI MegaRAID 320 2). However, the PCI-X, with a speed of up to 133 MHz and also 64 bits wide is becoming ever more widespread.
| PCI-X | 66 MHz | 100 MHz | 133 MHz |
|---|---|---|---|
| Bandwidth | 528 MB/s | 800 MB/s | 1064 MB/s |
In the end, what determines speed is the purpose for which the controller is used. The easiest way to calculate this is to multiply the maximum transfer rate of a single hard drive with the total number of hard drives. Eight Raptor drives each at 60 MB/s give a total of 480 MB/s. Even 66 MHz would be too little in this case because the new controller should also be able to serve future generations of hard drives at a sufficient rate. In this case we would prefer PCI-X with 100 MHz - although playing with figures around a RAID 5 does become very theoretical: Here, the architecture of a controller plays an equally significant role.
External Devices
An important benefit of SCSI has up to now been the available peripheral for operating any devices externally. Snazzy device superstructures are also possible since, depending on configuration, an SCSI bus can be up to 12 meters long. Although HighPoint offers serviceable external SATA solutions going under the name e.SATA, they do tend to suffer somewhat from the comparatively shorter cable length.
