SAS-Questions

SAS-Questions

The Answers

Will a SAS adapter will make a SATA disk drive run at 3 Gb/s?
If the SATA disk is only a 1.5 Gb/s SATA disk, 1.5 Gb/sec will still be the theoretical max. Other factors, such as disk rpm specifications will have a much greater impact on the array performance, as well as the number of disks in the array.

What type of host bus support is available with the SAS Adapters?
Only LSI supports SAS with both PCI-X and PCI express based HBAs and MegaRAID cards. This includes the PCI-X and PCI Express based HBA line, using native PCI-X and PCI Express controller chips. The MegaRAID line includes PCI Express based multi-port SAS options and a SAS PCI-X Zero Channel RAID card.

For SAS, LSI sees PCI Express as being the more popular interconnect going forward. This is because PCI Express eliminates the host bus performance bottleneck that PCI-X has and PCI Express based servers are becoming very poplular today.

Can I have a superfast (high spindle speed) SAS drive to boot from (say XP Pro) and a RAID 5 (4 SATA drives)to use as storage in the same machine?
Yes, because SAS and SATA drives share the same SAS backplane. This is one of the main benefits of SAS. You can configure “tiered” Storage by using one Atlas 15K.5 SAS for a fast bootup and configure multi-drive RAID with lower cost, high capacity SATA drives like Barracuda ES. Note that the server must have a SAS backplane.

What is SAS (Serial Attached SCSI)?
Serial Attached SCSI (SAS) is the logical evolution of the SCSI interface that satisfies the enterprise data center requirement of scalability, performance, reliability and manageability, while providing an enterprise storage infrastructure for both enterprise SAS drives and SATA disk drives. The SAS interface is compatible with SATA devices. This compatibility provides users with unprecedented choices for server and storage subsystem deployment.

Why was Serial Attached SCSI developed?
SAS was developed to address anticipated I/O and direct attach storage requirements in the future. It provides universal interconnect with SATA, while offering logical SCSI compatibility along with SCSI reliability, performance and manageability.

Aren’t parallel interfaces faster than serial interfaces?
In the past, parallel interfaces were preferable because their multiple data paths enabled greater throughput than the single data path of serial interfaces. However, recent advancements in VLSI technology have enabled serial transfer rates to make a huge leap forward, without the complex timing issues that hinder further parallel interface development. Compared to parallel SCSI, Serial Attached SCSI features higher throughput now and greater potential for advancement in the future.

What are the end user benefits of Serial Attached SCSI?
Key end user benefits include enterprise class robustness, investment protection in compatible SCSI software and middleware and the choice of direct-attach storage devices (SAS or SATA). In addition, greater performance, smaller form factors and greater addressability will all lead to a new level of flexibility when deploying mainstream data center servers and subsystems. Since SAS is based on the foundation of the industry-leading SCSI specification, reliability and peace of mind will satisfy user’s needs for continuity in the data center.

Does this mean parallel SCSI is now obsolete?
Not necessarily. Parallel SCSI has long played a key role in enterprise data storage, and will continue to have a presence. That said, Serial Attached SCSI is a compelling complement that matches the superb reliability and robustness of its acclaimed predecessor, while significantly expanding the SCSI envelope in terms of speed, scalability and flexibility.

Will the migration path from parallel SCSI to SAS be difficult?
Ease of migration was a primary consideration when Seagate and other industry figures collaborated to define Serial Attached SCSI standards. SAS was specifically engineered to leverage existing SCSI command sets, preserving your investment in storage management and enterprise application software. And unlike ATA solutions, no system-level workarounds are required to deploy SAS drives.

Is Serial Attached SCSI complementary to Serial ATA?
SAS complements SATA by adding dual porting, full duplex, device addressing and it offers higher reliability, performance and data availability services, as well as logical SCSI compatibility. It will continue to enhance these metrics as the specification evolves, including increased device support. SATA is targeted to cost-sensitive low work load non-mission-critical storage environments. Most importantly, these are complementary technologies based on a universal interconnect, where SAS customers can choose to deploy cost-effective SATA drives in a SAS storage environment. SAS allows the flexibility to install SAS drives, SATA drives or a mix of both SAS and SATA drives in the same enclosure, so long as the system is an inherent SAS system.

Are SAS and Serial ATA compatible?
Yes, compatibility with Serial ATA is indeed a core feature of Serial Attached SCSI. SAS backplanes and Host Bus Adapters (HBA) are fully compatible with Serial ATA, enabling connectivity for both types of drives on a common backplane, lowering infrastructure costs. SAS enclosures offer tremendous flexibility for growing businesses who can deploy Serial ATA drives when storage requirements are modest, then seamlessly add SAS drives as evolving business needs dictate. This guarantees the

What’s the difference between Serial Attached SCSI and Serial ATA?
Serial Attached SCSI is an enterprise-class solution that goes beyond its interface to deliver the superior performance, reliability and scalability demanded in mission-critical applications. Serial ATA is primarily a desktop-class solution suitable for use in light-duty environments where low cost is the highest priority.

What is a Serial Attached SCSI expander?
Expanders are the key to Serial Attached SCSI’s remarkable scalability. Each of these low-cost switches enables up to 128 point-to-point connections to be made off a single HBA/enclosure, and a total of 16,384 SAS devices can be aggregated while preserving performance and reliability. By contrast, parallel SCSI imposes a limit of fifteen devices per SCSI chain and severely limits total cable length.

Does SAS replace Fibre Channel?
Not at all. Serial Attached SCSI and Fibre Channel are both advanced, enterprise-class solutions with specific attributes that influence their suitability for a given application. SAS offers outstanding performance and exceptional scalability on a local level. Fibre Channel is well suited for large enterprise use [e.g., a Storage Area Network (SAN)] due to its support of up to 16 million addresses and maximum cabling distance of ten kilometers.

What are the power and cooling benefits of the Small Form Factor (SFF) 2.5″ drives?
SFF drives consume 70% less space and use 50% less power than the 3.5″ drives. The smaller form factor of the drives allows systems to be designed to accommodate more drives per U increasing the total system IOPs/U.

When should a customer use SAS HDDs?
Customers should use SAS drives in servers and bulk storage with mission critical and high work load intensive applications where 24/7 duty cycles are a requirement like database applications and high transaction level applications are the norm. SAS drives are intently designed around the attributes of performance and reliability.

I noticed LSI has released all of its MegaRAID controllers with a PCI express connector. The HBAs are currently only available with a PCI-X connector. Does LSI have any plans to release HBAs with a PCI express connector.
PCI Express offers a huge performance advantage over PCI-X with SAS. We saw this early and decided early to build our first SAS controller chip in a PCI-X flavor and a PCI Express flavor. Right now we are in the process of productizing 3 new PCI Express based HBAs that will be available in the coming months.

LSI advertises Integrated RAID on many of its HBAs. What sort of advantage does this offer over the alternative solution?
Historically HBAs provided lower level RAID 0 and 1 using software running off the host CPU. The issue with that is with PCI and PCI-X there is a huge bottleneck between the Host CPU and the controller for common RAID operations. This is an even bigger bottleneck in supporting RAID 10E. What LSI has done with the fusion architecture is utilized an embedded CPU core for all of the RAID operations as well as the controller driver layer. So LSI avoids the PCI bus for RAID functions and is able to use a very thin OS driver.

Are there any plans to increase the 8 meter SAS cable? This would seem to limit a large external network as compared to fibre or ethernet.
Currently the SAS specification allows cabling up to 10 meters. Eventually SAS cables will be able to reach 10 meters. In addition there should be repeaters available in the near future to exptend the overall cable distance.

What sort of qualification have the SAS controllers been through with different enclosures and drives?
LSI’s SAS HBAs and MR controllers are all being qualified by different enclosure suppliers using all available drives. In addition the Fujitsu FACT lab has done a considerable amount of testing with most of our SAS controllers using an array of enclosures and their HDDs.

Performance of SAS vs. SCSI. We sell a database system and have benchmarked best performance with SCSI drives at 30-40 users per spindle. Most of our customers have 75 – 200 users and typically wouldn’t require a high number of SCSI drives. It appears that SAS biggest advantage is with high numbers of disks. In this type of environment, would SAS offer a huge benefit?
Because of the serial interconnect aspect of SAS and the convenient cabling, SAS allows the ability for switching, which would have been too difficult with SCSI ribbon cables and fat connectors. LSI has been demonstrating a switch for this type of environment that will be available later this year.

How can multiple SAS domains be connected?
The SAS specification allows for this to take place in different ways. One way that LSI is handling this is through a SAS switch, available in Q3 2006. The switch will handle multiple domains dynamically for more flexible optimization.

Comparing SATA with SAS which is more reliable?
Where SATA drives offer cost and capacity advantages, SAS drives offer performance and more reliability.

Is SAS suitable for video playback?
Yes, Mutimedia and video playback benefit from the redundancy, reliability and overall performance advantages of SAS. What SAS will allow an end-user to do is handle a drive failure from the video source and switch to other redundant drives without interrupting the application.

Is the SAS command set identical to legacy SCSI command set, or are there additional SAS commands available?
Yes, the command set is identical. SCSI is organized hierarchically with an upper-level set of commands, that are passed down to an architecture layer and then from their down to the protocol layer. The SCSI Command set is not only implemented in parallel SCSI but also in Fibre Channel, SSA, Serial Bus Protocol, SCSI RDMA, iSCSI and SAS. So any topology or management differences between the parallel SCSI and SAS are managed at the protocol layer in the device drivers provided by the SAS controller, HBA and RAID suppliers and are masked from the upper level command set. As it turns out some of the upper level commands required in parallel SCSI aren’t necessary any more.

What kind of advantages does SAS bring to RAID in terms of data recovery and integrity?
SAS provides a number of advantages for RAID implementations in general:

  • Wide Ports, provide a greater degree of performance. By aggregating behind Wide Ports (4 SAS links for example) available bandwidth allows you a theoretical maximum of 24Gb/s full-duplex.
  • Secondly this available bandwidth allows you to scale to a very large number of drives without being bandwidth limited.
  • The bandwidth can also be an important factor in rebuild times for fail drives. While mostly limited by the drives performance, in terms of rebuild times, the high-bandwidth allows a more efficiently use the SAS links, leaving more time for other drives to share this resource.
  • Since all of this bandwidth can exist on a single PCI-X or PCI-Express controller, you can reduce the number of slots that are used in the server. Very important to many customers.
  • Since SAS supports both SAS and SATA drives, tiered storage can be implemented using common components for a range of application environments. Having a single RAID implementation that spans the SATA and SAS market spaces can be much more efficient in terms of servicing and lends itself to a higher quality RAID offering.
  • SAS also brings dual-porting, which allows for fail-over and data recovery, in shared storage environments. Port multipliers can be used in conjunction with SATA drives to deliver dual-porting support for SATA raid as well.
  • While just emerging in the market, SAS provides a capability for end-to-end error detection, which is important to unrecoverable write errors on the drives. This capability is often referred to as DIF.