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RPM
An 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 percent greater 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 have 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 can significantly affect performance. Most of the latest drives have at least 2 MB of cache memory, while others have up to 8 MB. Cache memory helps to deliver a performance increase over older drives with as little as 512 KB of cache memory.
Transfer rate
ATA/133 is available from some manufacturers. Consider the sustained versus the burst transfer rate.
Sustained versus Burst transfer rates
Sustained transfers are continued transfers that do not occur from the drive cache. Burst rates are data transfers directly to/from the high speed cache. A true indicator of performance is sustained rate; but, 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 26-42 MB/sec, depending on the drive. In a best-case scenario with the 2 MB cache full of data, 100 MB/sec quickly depletes the cache and commences the transfer at the lower sustained rate.
ATA/66 hard drives are typically less expensive and do not saturate the 66 MB/sec bandwidth available. A hard drive with a sustained transfer rate of 26 MB/sec will not even saturate available ATA/33 bandwidth.
In summary, consider sustained transfer rates more than 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). This technology addresses ongoing crosstalk and line noise concerns with PATA.
Serial ATA initially launched at 150 MB/sec transfer rates, Generation 2 drives launched at 300 MB/sec transfer rates while SATA III is expected to increase to 600 MB/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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