tkaczma@gryf.net wrote:
On Tue, 4 May 1999, Jaye Mathisen wrote:
Heck, I saw some very scary numbers from consensys for their IDE raid product, use IDE drives instead of scsi in this bad boy, and cut them costs down. WD makes 7200 RPM drives now, at 18GB's a pop, and 5 of 'em fit in a 3 high bay.
The price differences between SCSI and IDE are not that significant anymore, but I was wondering why someone didn't come up with and IDE appliance. The argument that I would come up with is that you can only have 2 devices on one bus, much too few for a RAID 5 set. This also applies to your request for a 3 disk appliance. The power of striping comes from the number of drives. The more drives you add the smaller the parity overhead. With 3 drives 33.3 ... % is "wasted" on parity. With 14 drives the "waste" goes down to about 7%.
(And no, I'm not interested in the philosophical war of IDE vs SCSI. Fact: IDE is currently cheaper than SCSI. Fact: I want this box to be cheap. Check and mate. :))
No mate, and probably not even check. SCSI drives are not THAT much more expensive anymore. You must remember that you can have only 2 IDE drives on the bus, among other limitations. The money you save in drives will probably be made up in the complexity of the controllers and software. Everything costs money, the drives are only a fraction of the cost.
Tom
The real issue with SCSI vs IDE is bus utilization.
The SCSI protocols were designed such that the bus could be freed for other users, in-between the time the request for data was sent to the drive, and the time data was returned. Thus commands for data could be sent to several drives, before the time the first drive could reply with the requested data.
The IDE protocols were designed for simplicity, it results in the bus being allocated to one request, for the duration of a single request. It wastes time for simplicity.
Due to the sporatic disk access behavior of workstations, IDE and SCSI drive perform similarly.
However, do to the much more consistant but varied disk access of file servers, the SCSI bus out-performs the IDE bus due to its time efficiency. The only mechanism to combat the IDE bus efficiency would be to have one IDE bus for each drive. Having one IDE bus per drive, would eliminates the minor cost advantage that IDE drives have over SCSI drives.
Note: Another issue that has not been mentioned, is system uptime. I have not seen an IDE drive that has been adapted to support hotplugging. This is critical for online maintenance.
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