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RPM
An often overlooked but extremely important hard drive feature is rotational speed, or RPM (revolutions per minute). As the hard drive spins faster, the data physically arrives at the read/write head more quickly. Based solely on RPMs, a 7200 RPM hard drive has a theoretical 33% performance improvement over a 5400 RPM hard drive!
Cache
Another important feature is the onboard cache. When a read command arrives at the hard drive, the drive first checks to see whether the requested data is in the cache. If the cache does not contain the requested data, the drive retrieves the data from the disk. Solid-state cache memory is thousands of times faster than the spinning platter of a hard drive.
A large cache will significantly impact performance. Most new drives have at least 2MB of cache memory, while some newer drives have up to 8MB of cache memory which helps to deliver a tremendous performance increase over older drives with as little as 512KB of cache memory.
Transfer Rate
ATA/133 is available from some manufacturers, but is it money well spent? Consider the sustained versus the burst transfer rate.
Sustained vs. Burst Transfer Rates
Sustained transfers refer to a continued transfer that does not occur from the drive cache. Burst rates refer to data transferred directly to/from the high speed cache. A true indicator of performance is sustained rate; however, most drives are advertised with their faster burst rate.
A typical ATA/100 hard drive bursts at about 100MB/sec from the cache, but has a sustained rate of about 26-42MB/sec, depending on the drive. If you consider a best-case scenario with the 2MB cache full of data, 100MB/sec will quickly deplete the cache and commence transferring at the lower sustained rate.
ATA/66 hard drives are typically less expensive and do not saturate the 66MB/sec bandwidth available. In fact, a hard drive with a sustained transfer rate of 26MB/sec will not even saturate available ATA/33 bandwidth.
In summary, sustained transfer rates should be considered over burst transfer rates for maximum hard drive performance.
Serial ATA
Serial ATA is the latest desktop hard drive technology. Serial ATA transfers data serially as opposed to the parallel transfers on traditional ATA drives (PATA, or parallel ATA), which addresses ongoing crosstalk and line noise concerns with PATA.
Serial ATA initially launched at 150MB/sec transfer rates, Generation 2 drives launched at 300MB/sec transfer rates, and SATA III is expected to increase to 600MB/sec. These are burst rates, not sustained transfer rates, but the sustained rate is higher than the typical PATA drive and has much more room to grow.
This applies to:
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