Compaq ProLiant 1000 DDS-3 Enhancing DAT Reliability, Capacity, and Performanc - Page 7

Faster Transfer Rate - parts

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ECG036/1297 7(&+12/2*< %5,() (cont'd) ... Other ways DDS-3 technology increases track space are by compressing sub-code information into less space and by packing user data into fewer, larger blocks, thereby reducing total overhead for data blocks. As a result of the increased linear bit density, format efficiency, and increased use of track space, DDS-3 has three times the track space that DDS-2 has available for data storage instead of timing, sub-code, Reed-Solomon ECC, and block overhead (Table 1). Table 1: DDS storage capacity comparison. DDS Version DDS-1 DDS-2 DDS-3 Native Capacity (uncompressed) 2 GB 4 GB 12 GB Compressed Capacity 4 GB 8 GB 24 GB Faster Transfer Rate DDS-3 has a maximum native data transfer rate of 1 MB/s (2 MB/s with compression), which is nearly double that of DDS-2. This increase is due in part to the change from embedded servo format to the more efficient timed tracking format. The increase in frequency/bit density also plays a part in the faster transfer rate of DDS-3. Because (1) the track is the same length for DDS-2 and DDS-3, and (2) there are twice as many bits in the same length of tape, the head passes over twice as much data in the same amount of time. Fast search is another feature that increases the data transfer rate. Fast search is the process of reading just the sub-code areas to locate particular data on a tape. The sub-code areas include information about user data such as group count, filemark count, setmark count, and block count, which allows the drive to find a particular data group quickly. The tape heads access the sub-code at a speed 100 times that of the normal read/write process. Normal tape speed for DDS-3 is 10.4 mm/s, but in fast search mode, DDS-3 tape speed is 1040 mm/s. During fast search, the rotational speed of the drum is slightly reduced so that the tape passes over the heads at the same relative speed. Because of the reduced drum speed, the tape head path is at a more acute angle than in normal read mode (Figure 6). Therefore, the head can only read a small amount of data from the track as it crosses it. To make this small piece of data usable, sub-code data is written in small, self-contained units among the user data. With DDS-1 and DDS-2, sub-code data is encoded on the track. However, with DDS-3, sub-code data is in the fragment headers (areas of the tape not intended for recording) and does not use space on the track that could be used to store data. Tracks Tape edge Head motion during fast search Normal head motion Figure 6: Normal and fast search tape head paths. 

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(cont’d)
ECG036/1297
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Other ways DDS-3 technology increases track space are by compressing sub-code information
into less space and by packing user data into fewer, larger blocks, thereby reducing total
overhead for data blocks.
As a result of the increased linear bit density, format efficiency, and increased use of track
space, DDS-3 has three times the track space that DDS-2 has available for data storage instead
of timing, sub-code, Reed-Solomon ECC, and block overhead (Table 1).
Table 1: DDS storage capacity comparison.
DDS Version
Native Capacity
(uncompressed)
Compressed Capacity
DDS-1
2 GB
4 GB
DDS-2
4 GB
8 GB
DDS-3
12 GB
24 GB
Faster Transfer Rate
DDS-3 has a maximum native data transfer rate of 1 MB/s (2 MB/s with compression), which
is nearly double that of DDS-2.
This increase is due in part to the change from embedded
servo format to the more efficient timed tracking format.
The increase in frequency/bit density
also plays a part in the faster transfer rate of DDS-3. Because (1) the track is the same length
for DDS-2 and DDS-3, and (2) there are twice as many bits in the same length of tape, the
head passes over twice as much data in the same amount of time.
Fast search
is another feature that increases the data transfer rate.
Fast search is the process of
reading just the sub-code areas to locate particular data on a tape.
The sub-code areas include
information about user data such as group count, filemark count, setmark count, and block
count, which allows the drive to find a particular data group quickly.
The tape heads access
the sub-code at a speed 100 times that of the normal read/write process.
Normal tape speed
for DDS-3 is 10.4 mm/s, but in fast search mode, DDS-3 tape speed is 1040 mm/s.
During fast search, the rotational speed of the drum is slightly reduced so that the tape passes
over the heads at the same relative speed.
Because of the reduced drum speed, the tape head
path is at a more acute angle than in normal read mode (Figure 6).
Therefore, the head can
only read a small amount of data from the track as it crosses it.
To make this small piece of
data usable, sub-code data is written in small, self-contained units among the user data.
With
DDS-1 and DDS-2, sub-code data is encoded on the track.
However, with DDS-3, sub-code
data is in the fragment headers (areas of the tape not intended for recording) and does not use
space on the track that could be used to store data.
Tracks
Tape edge
Head motion during
fast search
Normal head motion
Figure 6: Normal and fast search tape head paths.