What’s the relative positioning of Fibre Channel, ATA, Serial ATA (SATA), SCSI, and Serial Attached SCSI (SAS) in enterprise applications?
The good news is that OEMs, systems/storage integrators, and end users have more choices than ever when it comes to choosing disk drive/array interfaces. The bad news is that the plethora of options dictates a thorough examination of the strengths and weaknesses, and relative positioning, of the various options.
To address this issue, we deviated from our standard practice of not running articles written by vendors and invited the five leading (non-desktop) disk drive manufacturers to weigh in on the question of the relative positioning of the various enterprise-level interfaces. Participating vendors include Maxtor, Seagate, Fujitsu, Hitachi, and Western Digital.
We were curious to see if the responses would be characterized by consensus or controversy. For the most part, there was consensus-at least among four of the five vendors.
Bear in mind that disk drive manufacturers are a biased bunch. Although most of them strive to cover all bases-supporting all interface technologies, form factors, etc.-some vendors focus on specific technologies. Western Digital, for example, focuses squarely on Serial ATA disk drives. And each manufacturer has strengths or weaknesses in each market sector. Hence the inherent bias.
Although there appears to be a somewhat general consensus on where the various interfaces-and/or drive types-fit in the enterprise space, each manufacturer’s bias may become apparent on a close reading. In any case, the more opinions you weigh, the more informed a decision you’ll make.
What do you think?
We’ve published projections on drive/interface shipments from independent analyst firms many times in the past, but this time we thought we’d poll our readers on what types of drives they’re going to purchase over the next six months (see the figure, which represents results from September 2005 and December 2005 InfoStor QuickVote surveys).
Not surprisingly, Serial ATA topped the charts, with about half of our readers saying that SATA would represent the majority of their (non-desktop) purchases over the next six months. And as expected, interest in parallel interfaces (ATA and SCSI) is dwindling.
Serial Attached SCSI ranked a bit higher than expected, given the fact that very few SAS systems have made their way to end users at this time. The one surprise in our QuickVote poll was the staying power of Fibre Channel. In fact, the percentage of our readers choosing Fibre Channel as their primary drive type actually rose between September and December of last year, from 25% to 38% of the respondents. A survey conducted by Citigroup last year also indicated increased interest in Fibre Channel (see “CIO survey reveals surprises,” p. 10).
SATA may be hot, and SAS is coming on strong, but Fibre Channel is still a major player in the enterprise space. For opinions on the matter, read on. -Dave Simpson
OPINION: Match drive types to application requirements
By Kevin Wittmer, Maxtor
The migration of hard disk drives (HDDs) to serial host interconnects has resulted in a wide range of choices for system developers, integrators, and end users. Questions often arise as to what is the right choice of drive for a given enterprise application-whether it be internal server, direct-attached, or networked storage. Complicating the matter is the fact that the number and variety of enterprise storage applications has significantly increased recently, driven by factors such as compliance regulations, continuous 24x7 operations, disaster recovery, and management of the explosive growth in digitized content.
Choosing the appropriate hard drive for a given application is not a black-and-white decision and will depend upon factors ranging from budgetary constraints to performance and uptime requirements. However, general guidelines can be used to optimize drive choices.
Serial interface technology-Fibre Channel (FC), Serial ATA (SATA), and Serial Attached SCSI (SAS)-has facilitated the development of efficient multi-drive RAID systems. Gone are the complexities of routing wide buses and dealing with cumbersome ribbon cabling. Serial interconnects enable server and storage system developers to offer subsystems with a broad range of choices: FC, SAS, desktop SATA, and enterprise SATA (see table for summary comparisons).
Fibre Channel HDDs were the first drives with serial interfaces and have been available for nearly a decade. As such, Fibre Channel is the current standard for most external, networked storage subsystems. Today’s FC drives are available in capacities up to 300GB, provide high levels of performance, 24x7 power-on time operation, and high reliability at high I/O duty cycles, and enable high-availability systems with dual-port host interconnects.
Fibre Channel drives are best-suited for online and transactional applications in external storage systems. Typical applications include large databases, e-mail, Internet, and streaming data. Fibre Channel drives also preserve existing investments in FC-based SAN and NAS solutions as well as large monolithic storage arrays.
SAS, the successor to parallel SCSI, is the newest serial interconnect. Other than differences at the physical host-interface, SAS drives have virtually all of the attributes of FC drives with the added advantage that SAS-based systems provide seamless compatibility with SATA drives. This level of flexibility enables IT managers to reduce the number of different hardware platforms that need to be purchased, while enabling them to match appropriate drives to application requirements. For all types of transactional and online data storage solutions, ranging from internal server storage to external direct-attached storage and NAS, SAS drives are an ideal choice.
Desktop SATA drives are designed for the highest volume market (current estimate is more than 200 million units in 2005) and the lowest cost. The typical desktop SATA environment is a PC with one or two hard drives, running individual client applications with a nominal power-on time of 10 hours per day, five days per week. These types of drives provide capacities up to 500GB, with the bulk of the drives in the 40GB to 160GB range, and deliver good reliability in single-drive configurations at low I/O duty cycles.
Recognizing the opportunity to use inexpensive ATA drives in lower-performance, multi-drive configurations for applications such as bulk storage, disk-based backup, and archival and reference data, system developers started to utilize parallel ATA and SATA drives several years ago. Using generic desktop ATA drives provided mixed results, and the industry responded by making design and process improvements to enable the drives to operate better in multi-drive RAID configurations with low I/O duty cycles.
These changes included better tolerance to external vibration from adjacent drives, deterministic command processing times for RAID, and improved reliability in low duty-cycle workloads.
The result is a new classification of drive called enterprise SATA, which is currently the fastest-growing segment in the enterprise drive category. Demand for enterprise SATA drives will continue to grow with the ongoing demand for disk-based backup solutions to overcome shrinking backup windows, as well as the continual need for reference and archival storage systems to meet compliance and timely data recovery requirements.
With demands for storage capacity continuously increasing, IT managers are under pressure to get the most out of their limited budgets. To optimize the allocation of these funds it is important to understand the various drive choices and select the right drive for the intended application. IT managers will be able to create pools of storage with the optimal drive type(s) and leverage storage management software to move data across storage pools (e.g., online to reference to archival) as it changes over time.
Kevin Wittmer is senior director of technical marketing at Maxtor (www.maxtor.com).
OPINION: Positioning enterprise storage options
By Gianna DaGiau and David Szabados, Seagate Technology
Enterprise storage requirements are as varied and diverse as the enterprises they serve. In the case of interfaces for enterprise drives, the primary options are SCSI (both Ultra320 and Serial Attached SCSI, or SAS), Fibre Channel, and Serial ATA (SATA).
Ultra320 SCSI is the latest in parallel SCSI technology, which has been the primary choice of enterprises for the past two decades. However, Ultra320 SCSI is the end of the line for parallel SCSI technology because it would be technically very difficult to produce “Ultra640” SCSI.
SAS is the successor to parallel SCSI and provides the needed bandwidth and performance to scale for enterprises applications. SAS takes the best elements of existing SCSI-performance, reliability, and interoperability-and adds features and benefits that deliver new levels of performance and scalability that are unmatched by any other interface except for Fibre Channel.
Major system OEMs have already implemented SAS for their next-generation enterprise arrays and servers. High I/O transaction applications require an interface that can provide the needed bandwidth to handle the transaction traffic, as well as drives that deliver the highest performance and reliability. As a result, 15,000rpm SAS drives will become the mainstream choice for many enterprise applications.
SAS has additional benefits. In some data centers, IT managers need systems that use the least amount of space, enabling greater airflow and requiring less power, while having the highest transactional processing performance possible. Small-form-factor (SFF) 2.5-inch SAS drives meet these requirements.
Fibre Channel offers unparalleled flexibility to drive networks of storage across multiple data centers with the highest degree of bandwidth and system flexibility. The largest enterprises rely on storage infrastructures based on Fibre Channel.
The current 2Gbps bandwidth of Fibre Channel drives has begun to transition to 4Gbps. Advocates of replacement technologies for Fibre Channel cite the high costs associated with Fibre Channel systems, but fail to realize that initial acquisition costs are relatively meaningless compared to the total cost of ownership (TCO) over a system’s life. With large-scale systems, manageability and reliability (and the associated costs) become more important than initial acquisition cost. IT managers understand this. As a result, Fibre Channel remains the interface of choice for the most demanding systems.
The Storage Performance Council, a non-profit organization that disseminates vendor-neutral benchmark data, recently released the results of its SPC-2 benchmark tests, which are designed to measure sequential I/O performance (bandwidth) for storage systems. The results help IT managers and integrators estimate a system’s TCO, including the “performance cost per megabyte,” over time in real-world applications. All of the highest-scoring systems in the SPC tests were equipped with 15,000-rpm Fibre Channel drives.
SATA was designed as a replacement for desktop-class parallel ATA technology. Although not intended to serve as a primary enterprise interface, SATA is now being used in entry-level enterprise environments. This is a suitable solution for smaller systems that will not be required to scale or perform at the same level as SAS or Fibre Channel-based systems.
Another emerging trend is the deployment of SAS and SATA drives together in tiered storage systems. Interoperability and compatibility between SAS and SATA are designed in, as long as the base system is built around SAS (e.g., SAS controller into a server motherboard). For high-transaction data, SAS provides the ability to best serve client computers and workstations that take information off the server, while SATA drives can be used for storing less-frequently accessed data or bulk storage. This class of storage is often called “nearline” and narrows the gap between online and offline archival storage requirements.
Gianna DaGiau is senior global product marketing manager, and David Szabados is senior communications manager, at Seagate Technology (www.seagate.com).
OPINION: Serial interfaces highlight HDD market
By Mike Chenery, Fujitsu Computer Products of America
In 2005, the hard disk drive (HDD) industry achieved a number of technology milestones, with drives featuring new serial interfaces hitting the market in volume. (Another interesting technology milestone was the emergence of HDDs using perpendicular recording.)
This year will be marked by large-scale adoption of serial interface technologies, including Serial ATA (SATA) and Serial Attached SCSI (SAS). By year-end, 40% of all enterprise HDDs shipped are expected to use serial technology.
SAS will replace parallel SCSI, and a number of OEMs have already embraced the technology. SAS disk drives are readily available in the channel, and a variety of hardware vendors are selling SAS-based subsystems that can be configured with SAS and/or SATA drives. SAS drives are available in 3.5-inch and 2.5-inch form factors, with 2.5-inch small-form-factor (SFF) drives offering the advantage of higher densities and greater space efficiency.
Benefits of SAS include the following:
- High capacity and performance (both I/Os per second, or IOPS, and throughput);
- Greater flexibility, with the option of interoperability with SATA drives;
- Increased bandwidth scalability, using expanders as switches; and
- Improved reliability, with longer mean-time-between-failure (MTBF) ratings and high duty cycles.
Although SCSI will eventually be replaced by SAS, the other player in the enterprise drive category-Fibre Channel-is expected to maintain its place in the industry. Fibre Channel is an excellent choice for users that have invested in Fibre Channel infrastructure and have high-end enterprise systems that require exceptionally high performance. Fibre Channel will continue to hold its own, although it is not expected to take any market share from either parallel SCSI or SAS.
Another trend in the HDD market has been the penetration of high-capacity, low-cost 2.5-inch and 3.5-inch SATA drives for nearline storage. Storage and system integrators are looking at SATA drives as a cost-effective solution, especially in areas where the higher-reliability features of more-expensive enterprise drives are not required. SATA drives in the 2.5-inch form factor dissipate less heat and have lower power consumption compared to 3.5-inch drives. The appeal of 2.5-inch SFF drives is expected to grow because of higher densities, lower power usage, and more-deterministic performance since there is less data behind one actuator.
The introduction of perpendicular magnetic recording was a technology breakthrough last year in the disk drive market. Perpendicular recording is essential for HDDs to realize continued growth in areal density. Barring any new technological breakthroughs that will allow for increased storage density using traditional recording techniques, the industry is expected to transition to perpendicular recording.
During 2005, the disk drive industry reached the impressive milestone of shipping more than one million drives per day. The industry is expected to maintain a healthy growth rate this year, due in part to the rapid acceptance of new serial interfaces and SFF HDDs.
Mike Chenery is vice president, advanced product engineering, at Fujitsu Computer Products of America (www.fcpa.com).
OPINION: Serial interface technology on the rise
By Doug Pickford, Hitachi Global Storage Technologies
Use of parallel bus interfaces, which have long been the industry-standard storage interconnects, is declining. Parallel buses are increasingly being replaced by serial interfaces-Serial ATA (SATA) and Serial Attached SCSI (SAS).
The new serial interfaces address the bandwidth limitations of their predecessors. They support faster transfer rates and more devices per controller, as well as reduce the size and complexity of cables and connectors. SATA- and SAS-based systems will result in smaller, more densely packed disk arrays. Serial interface technology overcomes other parallel technology drawbacks as well, including signal skew, crosstalk, and limited device addressability.
Because SATA and SAS share the same physical connection, system builders can easily integrate both drive types on a single backplane. SAS controllers recognize both interface types and communicate with each device in its own “language.” This capability enables OEMs and integrators to design a single storage solution that meets a broad range of price, performance, and scalability requirements.
The configuration flexibility will result in systems that include a combination of SAS drives-to provide a high level of performance and reliability-or SATA drives, which provide high capacity and low cost for nearline storage, disk-to-disk backup, and similar applications.
Because SAS- and SATA-based drives can co-exist within a system, it’s important to understand each of their respective attributes and intended environments. The following is a brief overview of each serial interface and a set of recommendations for selecting a storage solution that is geared to meet your specific needs:
Serial Attached SCSI (SAS)
SAS builds upon established SCSI technologies typically used in RAID and other enterprise environments. SAS uses the existing SCSI protocol, while featuring serial point-to-point interconnections, dual porting, increased addressability, and full-duplex operation. SAS currently delivers a maximum data-transfer rate of 3Gbps, with a road map to 12Gbps. SAS also simplifies the design process for system builders because it uses simplified cable routing and shares similar physical attributes and interface transfer rates with SATA. The SAS feature set will enable development of high-performance, reliable disk systems that can be deployed quickly and easily.
In parallel architectures, multiple initiators have traditionally been used to provide drive access to multiple hosts and adapters. However, the multiple initiator approach can result in single points of failure that can prevent drive access. The dual-port characteristics of SAS eliminate single points of failure, enabling high-availability systems. This “full fail-over” capability, previously the exclusive domain of Fibre Channel, will increase reliability of direct-attached storage (DAS) architectures.
Serial ATA (SATA)
SATA was designed to replace parallel ATA in PCs, workstations, and other environments. SATA includes additional features for enterprise applications. SATA currently delivers a maximum data-transfer rate of 3Gbps, with future speeds of 6Gbps. The interface simplifies device configuration and improves interface-level data integrity. Like SAS, SATA drives use a compact cabling structure and smaller connectors, which enable enhanced air flow and reduce system complexity.
Choosing the right drive
There are several factors to consider when selecting a drive/interface for a given application. There are substantial differences between desktop and enterprise-class drives in terms of capacity, performance, reliability, scalability, and cost. SATA drives are most commonly used in single-user desktop applications because of their low cost-per-megabyte, extremely large capacities, and performance features.
In some cases, SATA drives can be used in “entry-server” applications. These are often DAS systems, populated with one or two drives. Low cost is a primary requirement for these systems, which are typically used in departmental or small business environments for local application serving.
Users are increasingly using high-capacity SATA drives for “nearline” or “fixed-content” storage, where 24x7 availability and low cost are necessary. Nearline storage is ideal for applications where data access is infrequent, such as disk-to-disk backup for long-term storage. When using SATA drives for nearline storage, users should look for drives that include technology that ensures drive performance in high rotational vibration environments, where there are multiple drives in a single enclosure. Other features to look for are native command queuing (NCQ) to enhance performance and staggered spin-up, which enables the host to individually “spin up” drives in multi-drive configurations. This reduces the power draw of a booting system, enabling system designers to reduce the size of the power supply and minimize total cost of ownership for users.
SAS drives will be used in enterprise environments (e.g., server storage, high-end workstations), where multiple users are accessing a single system. These mission-critical installations will require the highest levels of system performance for applications such as online transaction processing and intensive database queries.
Compatibility between the new serial interfaces benefits system builders and end users alike. The common serial interconnect will enable system builders to deploy systems that share common backplanes, connectors, and cabling. This greatly simplifies the process of changing the “mix” of drives within an enclosure, enabling users to easily replace a SATA drive with a SAS drive if their needs change.
Fibre Channel is another serial interface that is commonly used in high-end server applications and other multi-drive environments. Among the numerous benefits of Fibre Channel are a 4Gbps throughput rate, dual porting, and the ability to run extremely long cable lengths between drives and to hosts. When using fiber-optic connections, cable lengths can span nearly 10 kilometers.
Fibre Channel is also a “hot-pluggable” interface: Drives can be installed or removed while the host system is operational. This is a key factor in mission-critical environments that cannot afford system downtime.
With storage requirements increasing rapidly, flexibility and scalability will be critical to system builders and end users. The new serial interfaces provide users with configuration flexibility and investment protection. Serial interfaces have been designed so that as storage requirements change, integrators and users can easily transition between SATA and SAS drives without a significant infrastructure upgrade.
Doug Pickford is director of product planning and strategy at Hitachi Global Storage Technologies (www.hgst.com).
OPINION: The case for enterprise SATA
By Hubbert Smith, Western Digital
The results of InfoStor’s reader polls (see p. 22) indicate the general trends in the disk interface market today: Serial ATA (SATA) is red hot, Fibre Channel is holding its own, parallel interfaces (SCSI and ATA) are fading, and Serial Attached SCSI (SAS) shows promise.
- There are two distinct markets for enterprise drives: high capacity (SATA) and high performance (Fibre Channel, SCSI, and 10,000rpm SATA);
- The reliability of enterprise SATA systems is at an acceptable level. If these systems were unreliable, they would not be consumed in volume; and
- The combination of price, capacity, reliability, and performance of enterprise SATA systems is sufficient for many enterprise applications.
Parallel ATA (PATA) is the dominant interface in laptops and desktops. SATA is the replacement for PATA in server and external configurations and addresses the server-related limitations of PATA. Most server/storage systems that previously used PATA have already migrated to SATA.
SCSI is a legacy protocol and electrical interface, with many years as the leading mainstream drive interface for enterprise applications. Industry organizations such as the SCSI Trade Association and ANSI T10 group have developed a new standard-Serial Attached SCSI (SAS)-to replace parallel SCSI. The parallel SCSI drive and systems markets will shrink as SCSI is replaced by combinations of SAS and SATA.
SAS has several advantages over SCSI, including a point-to-point connection (each drive has an unshared, dedicated connection to the computer or controller). But the key advantage of SAS systems is that they can accept both SAS and SATA drives.
According to a joint white paper by Intel, LSI Logic, and Western Digital, “A robust SAS infrastructure, which supports both SAS and SATA drives without bridging, enables new options for delivering diverse storage solutions. IT managers are now able to mix drives within the same infrastructure, targeting different types of storage to different application requirements. This provides an opportunity for OEMs and integrators to leverage a common infrastructure for a broad range of end-user needs.”
Fibre Channel (FC) hard drives are high-performance drives, typically deployed in high-end storage arrays requiring connectivity to many servers. An interesting new development is the FC-SATA specification, spearheaded by Emulex, which expands FC capabilities to include compatibility with SATA disk drives (see “Emulex pushes FC-SATA spec,” p. 15).
The interest in enterprise SATA drives is due to the fact that they address key IT requirements, including very high capacity, solid reliability, and low cost.
The first SATA drives shipped in early 2003. The enterprise market valued the capacity points, but needed higher reliability than was offered in standard SATA desktop drives. As a result, SATA has evolved into three distinct classes: desktop, nearline, and enterprise.
Desktop SATA drives typically do not include server-level features. Nearline SATA drives are built for relatively low-duty-cycle applications and are typically limited to systems storing data that does not change frequently.
Enterprise SATA drives are built for the same environments as SCSI, Fibre Channel, and SAS. Enterprise SATA drives have comparable MTBF ratings at high duty cycles and include server-level capabilities such as the following:
- Queuing allows a computer to send many read-or-write requests, and the hard drive to re-order the reads or writes to the closest disk sectors;
- Dual-port (a.k.a. SATA Port Selector) capability allows two connections to each drive and enables external storage arrays with two controllers to both connect to each hard drive (if a controller fails, the other controller will continue to operate). This feature is also used in Fibre Channel and SAS;
- Error recovery coordinates error handling between drives and RAID controller; and
- Other enterprise SATA features include hot plug, staggered spin-up, microcode download, and asynchronous signal recovery.
In the past, SATA was incorrectly characterized as “a desktop interface lacking enterprise reliability.” That has changed. Enterprise SATA has shown that drive reliability is not determined by the silicon; instead, it is determined by the mechanical qualities of the drive and the associated manufacturing quality assurance.
Disk drive standards and technology have changed significantly, as has the business environment. IT has embraced the reality that most business data is mission-critical to some degree. Storage professionals understand the new reality that there are specific applications that require specific interfaces (or drive types) for performance and reliability, while other applications require capacity and reliability. SATA addresses the need for capacity and reliability, without the price tag of SCSI, Fibre Channel, or SAS.
Hubbert Smith is director of enterprise marketing at Western Digital (www.westerndigital.com).