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| FastSilicon's Guide To Storage Technology: Part 1 |
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Page 3 of 4 SCSI (Small Computer Systems Interface) Well well well, what can I say about SCSI that hasn't already been stated by every fan boy in the world? How about that SCSI is without a doubt, the fastest, most impressive disk based mass storage system on the market. Boasting a platter speed of anywhere from 7200 to 18000 RPM's, buffer sizes ranging from 2MB to 16MB, and a storage capacity from 9.1GB to 146.7GB, SCSI is THE platform if you are looking for extremely fast, high throughput storage options. All of these options come at an extremely large expense, however. The average SCSI disk is 74GB, and has a rotational speed of 15K rpms with a 16MB cache. The average price for one of these beauties? About $450. Kind of pricy right? Well, sort of. When you look at it from a usage point of view, the people who are buying SCSI storage are in need of storage that can put up with the demanding loads that SCSI was designed for in the first place. Well since I am rambling and drooling all over myself at the discussion of SCSI disks, lets dig deeper into why they are the 'kings of performance' shall we?  First on our list
is SCSI's blisteringly fast spindle speed. Modern SCSI disks have a
spindle of 15K or 18K RPM's, 15K being most common on dense storage
options, (36.7, 73.4, and 146.8GB disks), and 18K being common in the
lower densities (18.3GB, and 9.1GB). This intense speed enables the
read and write heads to seek to the location on the platter much
quicker than traditional 10K/7.2K IDE/SATA storage spindles, because
on a read/write miss (very unlikely) it takes much less time for a
sector to rotate back to its readable position again when it is
spinning at 15K rpms vs 10K or 7200.
Second up? Buffers. SCSI buffers are nothing special. There are 8MB and 16MB variants just like on SATA and EIDE, but the similarites end there. That is the ondisk buffer. The strength of SCSI storage lies in the SCSI controller, which most often has between 64MB and 1GB of ECC (error checking, correcting) memory on the controller. This enables the SCSI host controller to use its onboard memory as additional cache. Do you remember how we compared the 16MB buffer to the 2MB buffer and found a drastic read speed increase? Well, imagine if you had 1GB of cache and were sending 25MB files through the controller. First, the controller would tell the drive to seek to a location, read the data, and then it would get copied into the cache area of the SCSI controller after passing through the disk buffer. While it is in the SCSI controller's cache, the drive is free. Free to do what? Seek more data of course!. With a large controller based cache, SCSI disks can fetch massive amounts of data and cache them while fetching more. A function not implemented at all in SATA/EIDE controllers. This alone is the single most important feature that gives SCSI its performance crown.  Since I seem to
have rambled on and on about the greatest feature of the SCSI disk
subsystem, I think it is time to turn to the negative side of things.
Not everyone in the world has need for such a disk system, and
obviously, that can put a hurt in SCSI adoption. Less than 5% of
people that use computers need such raw disk performance to justify
spending oodles of cash on a disk system. So, for a price to
performance comparison, SCSI gets a 10. But on a price per usefulness
on average? SCSI gets a 2.5. Here's why. Even with capacities soon to
reach 300GB on SCSI interfaces, the price drives through the ceiling.
300GB SATA disks will cost you no more than $100, and a 300GB EIDE
drive will cost you about $65. If you went out and purchased a SCSI
300GB drive, and a controller for it, the total cost would be around
$900-1200, depending on the controller you purchased. That is an
insane amount of money for a single disk, and it has limited benefits
from a consumer standpoint.
Finally, since this guide is getting extremely long, I want to touch base on the last negative aspect of the SCSI storage path. That is of the SCSI controller itself. End user configuration of a SCSI host controller usually involves a hardware engineering degree from MIT or something equivalent. They are just plain problematic, getting the Logical Unit Number assigned properly, and having the OS pick it up w/o complaining. Stick to Serial ATA if you don't have plenty of cash or a demanding need. The headache is not worth it. I own SCSI storage, and I have to warn you about its good AND bad sides. |
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