The speed and size of the cache on the controller is of little importance when streaming multimedia data. The cache operates on the assumption that the system will want to access data in chunks smaller than the cache size thus allowing those very impressive burst transfer speeds that the drive interface likes to promise. The problem here is that streaming data will quickly overrun the cache and the interface transfer rate will quickly drop to the drive's sustained throughput rate. If the cache is VERY large, like 2 or more megabytes, then the drive's ability to "cache ahead" of the reads can be utilized, speeding up at least the first parts of the transfers. This is why some drives with very large caches but otherwise the same specs as their siblings are advertised as "multimedia" drives.

So now let's take a look at that ever important sustained throughput speed and the factors that effect it. First of all, let it be understood that when a drive is listed with a "disk to buffer" transfer rate, this is NOT the same as sustained throughput. It's at least a good marker, but not the same. Why is this, you ask? Well first of all, most of the time this disk to buffer transfer rate includes a lot of bits that are not data. Every sector contains error correction code (ECC) bytes that help the interface unscramble slightly mangled data without having to resort to a re-read of the sector. While these ECC bytes are very good to have, they don't contribute to the quantity of user data read. However, these extra bytes, usually 28 of them, are counted as part of the data transferred, and so make the numbers look higher. There are other bits on a disk that are counted in this number that don't make up any part of the user data and they too distort these numbers. Another point is that when only one value is given for this disk to buffer rate, it's almost always going to be under the most ideal conditions. This isn't realistic from a buyer's point of view, and so these numbers need to be taken with the appropriate quantity of salt. Only when a manufacturer lists explicitly the "sustained data throughput" or "sustained transfer rate" can you take these numbers to heart. This value will also take into account head switching and track-to-track seeking as well as latency and controller overhead. As a rule, the sustained throughput will be on the order of 60% to 70% of any advertised disk-to-buffer or buffer-to-disk rate. Said another way, the media to buffer rates will be about 1.5 times the sustained rates.

As you can see from the chart above, the sustained throughput is never as high as the disk to buffer transfer rate. However, if you know the sustained rate of one drive, you can compare its disk to buffer numbers with those of other drives and come to some intelligent conclusions.

There should be no question in anyone's mind that the faster a drive's rotation speed, the faster it can transfer data. It is safe to say that this one factor is the one single most important factor for segregating drives into classes. As mentioned above, higher rotation speeds allow the target sector to become positioned under the heads faster and the bits can be written or read faster. In the same way, the drive with high areal density will translate directly into a drive with faster throughput. Simply put, the more data that can be packed into a track, the more data can be transferred without moving or switching the head and the faster the bits will go through the head. The disk to head transfer rate is a product of density and rotational speed. It's clear that if in the same revolution of the platter, you read more data, you're reading it faster. Large capacity drives with few heads are going to be faster then drives with the same capacity but more heads and the faster the rotation speed, the quicker the throughput. It's that simple.

There is one point that must be made about the faster 7200 and 10,000+ rpm drives. They can be VERY noisy! This could be a consideration if your computer is in the same room where you like to track. Special computer boxes are available to keep the system cool while muffling the noise, but be prepared to spend for them. If you attempt to build your own "Quiet Box" for your system, be sure to provide good air circulation. There's nothing gained by a quiet computer that keeps shutting itself down right in the middle of that "golden" take! Luckily, newer 7200 rpm drives are much quieter then the the earlier ones.

A consideration that may not occur to many folks just of the top of the head is how cooling will effect drive performance. The faster the rotation speed of a drive, the hotter it will get. Also, as a rule, SCSI drives tend to run hot anyhow. This is why many SCSI drives recommend drive fans or some form of passive drive cooling. If a drive is allowed to overheat, this not only puts stress (perhaps fatal stress) on the electronics, but can cause the data storage media to fail, and there goes you big hit record! This isn't to say that IDE drives are immune to this consideration. These drives get hot too, especially the 7200 rpm jobs. Everyone knows that good cooling will keep the CPU happy, even if you're not overclocking. Good cooling will keep your drive happy too. For starters, don't mound the hard drive at the bottom of the internal bay unless the bottom of the bay is open to the inside (uses mounting flanges instead of a solid metal bottom). Keeping a gap under the drive will promote good air circulation. Add a second fan if you don't have one already. A fan in the front of the chassis blowing in will complement the power supply fan in the back blowing out.

Another factor of drives and heat is the phenomenon of thermal recalibration. This is a tendency of drive controllers to detect that the platters have expanded due to heat. In response to this determination, the controller will home the heads and launch them to a specific spot and calculate the error between where it thinks the heads should be and where they ended up. It then re-calculates the predicted locations of the tracks on the disk using this data. In doing so, the controller has a much better chance of hitting its target track on a seek if it knows how much that track has moved out of position due to thermal expansion of the platter. Yes, this is a good idea and a cool thing to do - except during a recording or mixing session. The thermal recalibration takes a large fraction of a second to perform and that's more than enough time to stall out a streaming operation. A recal is NOT the kind of thing you want your drive to be doing in the middle of a recording session! Keeping the drive cool and at a stable temperature is the best defense against this happening.

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