3Ware TwinStor Fiche d'information - Page 4

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Drives are usually thought of as random access devices. Any drive must seek to position the actuator and wait for the

desired data to rotate until it is under the head. Seeks to nearby tracks are much faster than seeks to distant tracks, and

seek times can vary from a few milliseconds (ms) to a few tens of ms. The rotational latency depends on the RPM of the

drive, with 5,400 RPM, 7,200 RPM and 10,000 RPM drives having maximum latencies of 11.1 ms, 8.3 ms and 6 ms

respectively. During sequential accesses within a track, there is no waiting for rotational latency because the needed data is

already under the read head. When a sequential access extends beyond a track, a delay of a few ms is required to switch

heads. Data is formatted with some skew so the read head is automatically in position to retrieve the next sequential data

upon completion of the head switch – this eliminates an entire revolution of the disk that would otherwise be necessary to get

to the position of the data. Reading data sequentially can be orders of magnitude faster than reading the data with short

random accesses.

The basic idea behind 3ware's TwinStor technology is to reduce seek times and avoid rotational latency by using intelligent

algorithms executed by the embedded microprocessor on the disk switch. The high-level flowchart in Figure 1 shows the

separate profiling and execution steps.

Figure 1. TwinStor Technology Flowchart

Profiling

The first time a new disk is encountered by the storage switch, the profiling program scans the disk to find the zone breaks,

the number of tracks per zone, and other performance information. The result of the profiling is stored in a zone table in a

small reserved section on each drive. During execution, the storage switch records an access history to determine whether

the current request is best considered a sequential or random access. Separate optimization techniques are applied to these

two types of accesses