SATA II: What you should know
By Greg Schulz
When you hear the term SATA, you may think of 3 gigabytes per second (Gbps) data transfers rates, port multipliers, SAS and SATA co-existence, among others. If so, you are not alone. SATA II means different things to different people and it's not just the next iteration of SATA and successor to parallel IDE/ATA technologies.
Features of SATA II devices and components (disk drives, adapters, port multipliers and selectors, cabling and connectors) support 1.5 Gbps and 3 Gbps data transfer rates, along with some combination of features (asynchronous notification, ClickConnect, hot plug, power management, NCQ and staggered device spin-up, among others). Some examples of SATA II compliance implementations include:
|Device or component||Interface speed||Additional feature|
|Disk drives||3 Gbps||NCQ, Hot plug, staggered spin-up|
|Port multipliers and selectors||3 Gbps||NCQ, staggered spin-up|
|Cabling and connectors||up to 3 Gbps||SATA, eSATA, xSATA|
NCQ is a performance enhancement for SATA II-category disk drives, and works similarly to the way command tag queuing (CTQ) works in SCSI command set-based disk drives (e.g. Fibre Channel (FC), SAS, UltraSCSI, etc). NCQ addresses the inherent inefficiencies, along with wear and tear on a disk drive due to excessive read/write head movement caused by random I/O patterns. As more I/Os arrive from more sources -- either different applications or different sources along with random I/O patterns in general -- this excessive disk head movement becomes more of an issue. As additional I/O operations arrive from different sources in a multi-stream, multi-I/O environment, more disk head "thrashing" can occur. To help cope with this issue, NCQ algorithms allow I/O operations to be performed out of order to optimize and leverage disk read/write head positioning and ultimately overall performance.
Using port multipliers, product providers can increase the number of SATA disk drives that can attach to a single controller port. The port multiplier enables multiple SATA devices (disk drives) to fan-in (converge) to a single controller port, similar to how a switch or hub enables multiple devices to be accessed via a single port. This benefit is similar to that of FC, SAS, and UltraSCSI storage interfaces, but on a smaller scale in order to be able to have more than a couple of disk drives per SATA controller interface. For example, SATA II supports up to 15 disk drives per controller interface, compared to the relatively small two disk limit of ATA master/slave configuration.
SAS and SATA co-existence
Not to be forgotten or lost in the discussion is the ability of SATA II devices to co-exist with SAS devices attached to a common SAS-based backplane and SAS-based controller interface. This is unique in that storage interfaces usually need to be attached to like interfaces: for example, SATA to SATA, FC to FC, SCSI to SCSI, SATA to SATA. SATA II, however, has the ability to co-exist and attach to a SAS-based controller and SAS-based backplane interconnect. The reverse is not supported, meaning that SAS devices cannot plug into SATA controllers. The benefit is that some high-performance SAS disk drives could be configured for low latency, high performance I/O, while SATA disk drives configured for storage and capacity-centric applications in the same disk shelf.
As with all good things, there is a caveat here to be aware of: Similar to mixing different types of disk drives in a storage solution today (e.g. FC and SATA or PATA), you will want to make sure your RAID and storage groups are configured properly. For example, avoid configuring storage pools based on what available capacity is left; instead, keep SATA disk drives grouped together and SAS or FC-based disk drives of the same performance class grouped together when creating RAID, storage and volume groups. By keeping like disk drive types configured together, you should avoid potential performance challenges.
Speaking of disk drives, let's clear up some confusion and a common myth about SAS disk drives. The myth or perhaps misunderstanding is that the new 2.5" small-form factor disk drives will only be SAS; or, put another way, all 2.5" disk drives are SAS. The reality is that 2.5" disk drives, like the existing 3.5" disk drives, will be available with different storage interfaces (e.g. FC, SAS, SATA and UltraSCSI). The Seagate Savvio is an example of this.
Today there are 3.5" disk drives with FC, SAS, SATA and UltraSCSI storage interfaces, and moving forward, there will be new 2.5" disk drives to support multiple storage interfaces like FC and SAS, as well as different levels of performance, capacities and reliability objectives.
SATA technology is maturing on the upper end to take on more responsibilities in the enterprise. You can learn more about SATA, including when, where and how to implement in the Ask the Experts section of SearchStorage.com, which includes "Tiered storage with SATA" and "Myths and Realities of SATA".
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About the author: Greg Schulz is a senior analyst with the independent storage analysis firm The Evaluator Group Inc. and is the author of "Resilient Storage Networks."
05 Oct 2005
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