Compaq ProLiant 3000 DDS-3 Enhancing DAT Reliability, Capacity, and Performanc - Page 5
Advantages Of Dds-3 - price
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ECG036/1297 7(&+12/2*< %5,() (cont'd) ... ADVANTAGES OF DDS-3 In comparison to DDS-2, DDS-3 technology provides significant improvements in capacity, transfer rate, and reliability. When considering all the new features, the drive price, the media price, and the overall reliability, it is easy to see DDS-3 also offers a low total cost of ownership. Increased Capacity Although DDS-3 cassettes use almost the same length of tape as existing DDS-2 cassettes, DDS-3 has three times the native capacity (12 GB vs. 4 GB). Factors contributing to this increased capacity include: • Increased linear bit density • Increased format efficiency • Increased use of track space ,QFUHDVHG /LQHDU %LW 'HQVLW\ DDS-3 was originally intended to have a native capacity of 8 GB, twice that of DDS-2. However, the development team discovered that the linear bit density could be increased more than they originally thought. In fact, the density was doubled from 61,000 bits per inch (bpi) to 122,000 bpi. While this doubling of the linear bit density does not result in double the capacity, this increase does significantly impact the capacity of DDS-3. The capacity of a tape is almost doubled with data compression, depending on the nature of the data being compressed. Typically, text can be compressed at a 2:1 ratio, while graphic files can be compressed at a 5:1 ratio. However, a file that is already compressed will not compress further during the backup. ,QFUHDVHG )RUPDW (IILFLHQF\ DDS-3 also has increased format efficiency over DDS-2 due to Partial Response Maximum Likelihood (PRML). PRML is a technique used to differentiate a valid signal from random noise. Noise results in data errors. Because DDS-3 has PRML, it incurs fewer data errors due to random noise. ,QFUHDVHG 8VH RI 7UDFN 6SDFH In multiple ways, DDS-3 technology increases the amount of track space that can be used to store data. First, DDS-3 uses the timed tracking servo format instead of the embedded servo format used in DDS-1 and DDS-2. Servo tracking is a scheme used to regulate positioning and seeking operations in disk and tape drives. This scheme relies on positioning data recorded on the media surface to generate correction signals for the servomechanism. A system with an "embedded" servo, as in the Compaq 2/4 GB DAT and 4/8 GB DAT drives, uses the automatic track following (ATF) method to ensure the tape head is in the center of the track being read. There are ATF areas embedded at the beginning and end of each track; user data is recorded between them. The read and write heads are wider than a single track, so the head reads the ATF areas from the two tracks on either side, as well as that of the track the head intends to read. By comparing the content and strength of the signals from the ATF areas, the servo system adjusts the drum and tape and allows the head to pass directly over the centerline of the track being read (Figure 4). The ATF areas of the track cannot be used to store data.