Comparing SCSI and ATA drives

Bits & Bytes: Author of "The Backup Book" continues his comparision series by looking at SCSI vs. ATA drives.

There's pretty much no difference in interfaces or bandwidth between any of the SCSI or ATA scenarios as both use DMA to transfer data to and from the CPU. Serial ATA is reaching the speeds of Ultra SCSI, and every format except the older parallel ATA are hot pluggable, with both serial ATA and SCSI supporting both internal and external formats.

  Parallel ATA USB FireWire 800IEEE 1394 Serial ATA SCSI
Connectivity Market Internal storage External storage External storage Internal and External storage Internal and External storage
Cost comparison Base > Parallel > Parallel = Parallel > Parallel
Speed 100 60 100 150 160
Bootable (Win) Yes No No Yes Yes
Bootable (OS X) Yes Yes Yes Yes Yes
Hot Pluggable No Yes Yes Yes Yes
Cable Lgth (m) .45 6 (per link) 4.5 (per link) 1 12

SCSI and ATA comparisons
There is a significant difference in price and warranty between the two though. Let's look at the differences between an ATA and SCSI drive from the same manufacturer -- Maxtor.

Type Capacity Speed Price SMART Warranty
DiamondMax ATA 80.0 GB 133 MBs 88.24 Yes 1 year
Atlas 10K III SCSI 73.4 GB 160 MBs 279.77 Yes 5 years

Individual drive comparison
Funny enough, the capacity is slightly smaller on the SCSI drive than the ATA drives (which is due to formatting differences between the two). The ATA drive is slightly slower than the SCSI drive. If the differences were only these two items, I'd say it was a wash. But, the real differences come in with the price and the warranty.

Any sane network administrator is going to have at least 1 spare drive for every 10 drives in use so that when one breaks, the spare can be grabbed from the shelf and immediately put into use while the original is being shipped back under warranty.

To even things out between the warranty differences of one and five years, we'll say that the administrator will have to replace each of the ATA drives 1.5 times over the five year period.

Type Originals Spares Replacements Cost
SCSI 10 drives 1 spare 0 $3,077.47
ATA 10 drives 1 spare 15 $2,294.24

Price differentials
Even with the drive replacements, the costs of the ATA drive format are still less over a five-year period than the cost of the SCSI drives. And that's just for single-drive implementations. What about mirrored drives or RAID arrays? If we extend the argument above, this time adding the additional drives for mirroring or RAID 5, plus the hot spare for the RAID 5 system, we can still see that the ATA arrays are going to be less expensive (even allowing for failure) over the long run.

Type Originals Hot Spare Add'l Spares Replacements Cost
Mirrored SCSI 2 drives -- -- -- $559.54
Mirrored ATA 2 drives -- -- 3 $441.20
RAID 5 SCSI 5 drives 1 -- -- $1,678.62
RAID 5 ATA 5 drives 1 -- 9 $1,411.84

RAID price differentials
So what's the real difference? You'll be replacing more drives more often. Which means that you'll want to know a lot more than you know now about SMART reporting (our next article) and drive management strategies.

For more information:
 

Back to part one, "Serial ATA vs. parallel ATA"

For additional information on serial ATA:

This was first published in November 2003

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