Fall 2003 IDF

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Think Smart, Think SATA II

Fortunately, focusing on improving the implementation has lead to quite a few advances over prior storage standards. The SATA II standard expands the physical layer, from two to six different types as indicated by Table 2. Gen1m is an extension of Gen1i, intended to be use for storage arrays and enclosures. Due to the signaling characteristics of the longer interconnect; Gen 1m receivers are more sensitive to noise than their Gen 1i counterparts. Unfortunately, this means that Gen 1m receivers may not be usable in a less lossy environment without extra circuitry to regulate the signal level.

Table 1 – Serial ATA Physical Specifications

SATA will be augmented in the future by AHCI (A Host Controller Interface), which will help to provide many of the tertiary benefits of SATA II. AHCI will provide the driver and software support necessary for the implementation of SATA’s hot swapping capabilities, as well as the power saving features that are needed in mobile environments. The power savings are realized by enabling both the hardware (controller) and software (host) to shift the link and interface into one of the two power saving modes: slumber or partial. So, the controller could enter into partial mode while the head is seeking a new position on the platters, and the host could go into slumber when it knows that there are no more requests to be filled.

SATA II also incorporates advanced queuing functions, previously only found in SCSI drives. The SATA II extensions allow for up to 32 outstanding commands for a single device, as well as re-ordering of requests and out-of-order data delivery [8]. In some sense, this can be viewed as a migration of out-of-order execution even further into computer systems, from the MPU to other subsystems. This is extremely advantageous, particularly when paired with the more powerful microcontrollers that can intelligently optimize queues. Queue re-ordering and intelligent optimizations will reduce the number of disk rotations per data unit transferred and also improve disk latency. Ultimately, it should come as no surprise that many of the performance features incorporated into SATA were previously found in SCSI. Intel itself indicates that the queuing features in SATA are “conceptually similar to SCSI tagged command queuing.” [5]

In addition to providing performance oriented features in SATA II, standards for many other types of SATA devices have been established. Most of these auxiliary devices are intended to create a more flexible and robust SATA infrastructure, and to push SATA into higher end storage solutions than its predecessor. The simplest such device is the port multiplier, which allows up to 15 devices to attach to a single host. Only a single port multiplier may be used with a given host, so it is not possible to create sub-networks of SATA devices. This can be used in conjunction with a 4 channel SATA interface and connector to provide up to 1.2GB/s for a set of devices. There is also a standard for port selectors, which has two attached sets of devices. The port selector has an exclusive switch which can only use a single set of devices at a time. Port selectors can be used with port multipliers, which allows for many interesting applications. For example, a host could attach through a port selector to two different sets of port multipliers, each with 15 attached drives. One set would be used in normal operation, while the other would be in place for fail-over. Alternatively, to achieve a finer granularity, a port multiplier could attach to 15 port selectors, each of which would have two attached drives.

All together, it is clear that the designers of SATA II intended it to not only take over desktop and mobile systems, but to also invade a niche previously held by SCSI: enterprise servers and datacenters. The SCSI Trade Association, which many of the larger SATA members also belong to, has obligingly designed Serial Attached SCSI (SAS), which allows the SCSI software stack to operate on top of the SATA physical and electrical specifications. It is likely that in the next few years, this combination will become extremely popular in low to midrange servers, as it inherits the RAS features of SCSI drives and the cost structure of SATA devices.

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