4.5.2Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that which was requested by a Read command are retrieved and stored in the buffer for immediate transfer from the buffer to the host on subsequent Read commands that request those logical blocks (this is true even if cache operation is disabled). Though the prefetch operation uses the buffer as a cache, finding the requested data in the buffer is a prefetch hit, not a cache operation hit.

To enable Prefetch, use Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch.

Since data that is prefetched replaces data already in some buffer segments, the host can limit the amount of prefetch data to optimize system performance. The Max Prefetch field (bytes 8 and 9) limits the amount of prefetch. The drive does not use the Prefetch Ceiling field (bytes 10 and 11).

During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if Mode parameters page 08h, byte 2, bit 4 is set to 1 (Discontinuity--DISC bit).

When prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA (Adaptive Read Look-Ahead). If the host uses software interleave, ARLA enables prefetch of contiguous blocks from the disc when it senses that a prefetch hit will likely occur, even if two consecutive read operations were not for physically contiguous blocks of data (e.g. “software interleave”). ARLA disables prefetch when it decides that a prefetch hit will not likely occur. If the host is not using software interleave, and if two sequential read operations are not for contiguous blocks of data, ARLA disables prefetch, but as long as sequential read operations request contiguous blocks of data, ARLA keeps prefetch enabled.

4.5.3Optimizing cache performance for desktop and server applications

Desktop and server applications require different drive caching operations for optimal performance. This means it is difficult to provide a single configuration that meets both of these needs. In a desktop environment, you want to configure the cache to respond quickly to repetitive accesses of multiple small segments of data without taking the time to “look ahead” to the next contiguous segments of data. In a server environment, you want to configure the cache to provide large volumes of sequential data in a non-repetitive manner. In this case, the ability of the cache to “look ahead” to the next contiguous segments of sequential data is a good thing.

The Performance Mode (PM) bit controls the way the drive switches the cache buffer into different modes of segmentation. In “server mode” (PM bit = 0), the drive can dynamically change the number of cache buffer segments as needed to optimize the performance, based on the command stream from the host. In “desktop mode” (PM bit = 1), the number of segments is maintained at the value defined in Mode Page 8, Byte 13, at all times (unless changed by using a Mode Select command). For additional information about the PM bit, refer to the Unit Attention Parameters page (00h) of the Mode Sense command (1Ah) in the SAS Interface Manual, part number 100293071.

Savvio SAS Product Manual, Rev. D

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Seagate ST936701SS, ST973401SS manual Prefetch operation
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ST936701SS, ST973401SS specifications

The Seagate ST936701SS and ST973401SS are high-performance enterprise hard drives designed for optimal data storage solutions in demanding environments. Both models belong to Seagate's Savvio series, which is renowned for its reliability and efficiency. These drives are tailored for critical applications such as database management, data warehousing, and online transaction processing.

The ST936701SS comes with a storage capacity of 36.4 GB, while the ST973401SS offers a larger capacity of 73.4 GB. This variance allows users to choose the drive that best suits their storage needs without compromising performance. Both drives utilize a 2.5-inch form factor, making them compact and suitable for high-density storage configurations.

A key feature of these drives is their impressive rotational speed of 10,000 RPM, which enhances data access times and improves overall system responsiveness. This speed allows for reduced latency and faster data transfer rates, critical for applications that require quick retrieval of large datasets.

In terms of technology, these drives utilize the Serial Attached SCSI (SAS) interface, which is favored in enterprise settings for its reliability and speed. SAS provides better performance than traditional SATA drives, particularly when dealing with high workloads, as it supports multiple concurrent connections and higher data throughput.

The ST936701SS and ST973401SS are also equipped with advanced features such as Seagate's Native Command Queuing (NCQ), which optimizes the order in which read and write commands are executed. This results in improved performance under multi-tasking conditions, essential for enterprise servers managing multiple requests simultaneously.

Additionally, both drives incorporate features aimed at enhancing data integrity and reliability. They support End-to-End Data Protection and are designed to endure the rigors of continuous operation, with MTBF (Mean Time Between Failures) ratings that bolster their reputation for durability.

Energy efficiency is another notable characteristic, as both drives are designed to reduce power consumption without sacrificing performance. This is particularly important in enterprise environments where power management contributes to lower operational costs.

In conclusion, the Seagate ST936701SS and ST973401SS drives are robust, reliable storage solutions tailored for enterprise applications. With their high performance, advanced technology features, and capacity options, they provide organizations with the scalability and efficiency required in today’s data-driven landscape. Whether it's for critical data management tasks or high-access applications, these drives stand out as a solid choice for any enterprise storage strategy.
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