In: Root » Computers and technology » Storage Devices
A new class of hard disks has been developed that has direct application to multimedia and, in particular, authoring. Called AV drives because their primary application is in the storage of audio and video data, these drives are optimized for the needs of those applications. In general, they are large, starting at one gigabyte and climbing skyward from there so that they can store workable amounts of high quality data. More important than capacity is speed. Unlike normal hard disks that must provide instant access to completely random data, AV drives are designed for high speed sequential access. Audio and video files tend to be long and linear, read sequentially for long periods. Access time is not as important for such applications as is a high sustained data transfer rate. For example, most video production today requires data rates of 27 megabytes per second but use compression ratios averaging about 10 to 1 to produce a data stream of about 2.7MB/sec that needs to be stored and retrieved. To achieve the highest possible performance, they add extensive buffering and may sacrifice or delay some data integrity features. In order to handle AV data as fast as possible, AV drives generally have very high rotational rates, some up to 7200 RPM, and newer drives will undoubtedly climb higher. In addition, most include large embedded buffers so that they can maintain high data throughputs even when incrementing between tracks and encountering errors. AV drives also attempt to maintain data contiguity, keeping sequential sectors in the data stream together so that head movement can be minimized. To achieve the high transfer rates required by video applications, AV drives need to use every speed-up trick they can find. These run a wide range, and most involve trading off absolute data security to eliminate interruptions in the high speed flow of information. The hard disk industry rationale for this design is that video data, unlike spreadsheets or databases, tolerates errors well so a short sequence of bad data won't hurt anything. After all, a single sector isn't even a third of a line of uncompressed video. The video subsystem can correct for such one-time errors in part of a line, masking them entirely. One way to prevent interruptions in the flow of data is to alter error handling. Engineers usually require that most hard disk drives attempt to re-read the disk when they encounter an error. Most manufacturers use elaborate algorithms to govern these re-reads, often minutely changing the head position or performing an entire thermal calibration. If these are not successful, the drive may invoke its error correction code to reconstruct the data. AV drives alter these priorities. Because error correction is entirely electronic and imposes no mechanical delays, AV drives use it first to attempt error recovery. Only after error correction fails may the drive try to re-read the data, often for a limited number of retries. Advanced hard disks log the errors that they encounter so that they may be used later for diagnostics. The logging operation itself takes time and slows disk performance. AV drives delay error logging until it does not interrupt the data stream. One feature that slows down conventional drives is sector re-mapping, which AV drives avoid to eliminate its performance penalties. The re-mapping process imposes delays on the flow of data because the read/write head of the drive must jump from one place to another to write or retrieve the data from the re-mapped sector at the time of the data access. AV drives often avoid auto-relocation to prevent interruption of the flow of high speed data. The possibility of uncorrected errors occurring in the reading and writing of your data makes AV drives unsuitable for critical data applications. When used for their design purpose, however, AV drives deliver excellent performance with reasonable levels of risk.
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