Serial Attached SCSI (SAS)
Serial Attached SCSI (SAS) interface is the successor to SCSI (Small Computing System Interface). The major enhancement is that SAS is a point-to-point interface allowing each device on a connection to have the entire bandwidth available. The current bandwidth of each SAS port is 3Gb/sec. with future generations aimed at 6Gb/sec and beyond.
Parallel U320 SCSI bus versus point to point Serial Attached SCSI connection Other key benefits to SAS interface:
- SAS interface protocol supports both SAS and SATA Hard Disk Drives allowing tiered storage management.
- SAS supports seamless scalability through port expansion enabling customers to daisy chain multiple storage enclosures.
- SAS supports port aggregation via a x4 wide-link for a full external connection bandwidth of up to 12.0 Gbps (1200MBps) on a single cable and a single connection.
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Keep in mind:
- SAS is not backwards compatible with U320 SCSI or previous SCSI generations
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Serial Attached SCSI (SAS) Hard Disk Drives
SAS hard disk drives (HDDs) matched with a SAS interface can maximize the performance of the storage array. With three drive speeds, 15K, 10K and 7.2K RPM customers can choose the right mix of performance, reliability and capacity for their application.
Both 10K and 15K RPM drives offer superior MB/sec performance with reduced latency over nearline (7.2K RPM) drives. While higher RPMs have the edge on MB/sec and rotational latency, there has been a trade off in capacity. Nearline SAS HDDs maintain all of the benefits of the SAS interface while also bringing the high capacities and dollar per gigabyte advantages of SATA HDDs.
10K and 15K SAS HDDs has been rated at 1.6 million hours MTBF (mean time between failures) thus delivering superior reliability over SATA which is rated at 1.2 million hours MTBF. Nearline SAS has been rated at the same MTBF as SATA, but offers considerably better performance.
Once the decision has been made to utilize SAS HDDs, priorities around reliability and performance versus dollar per GB will drive the choice for 7.2K RPM SAS or traditional 10K and 15K RPM offerings.
More Benefits of SAS HDDs:
- SAS HDDs are true dual port, full duplex devices. This means SAS HDDs can simultaneously process commands on both ports.
- All SAS HDDs are hot-swap capable. Users can add or remove an HDD without disrupting the enterprise environment.
- Most Dell SAS HDDs support online firmware update. This allows users to update firmware on the SAS HDD without having to schedule downtime.
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Keep in Mind:
- SAS HDDs cannot be used on the older architecture SCSI backplanes or cables.
- SAS HDDs typically draw more power than the equivalent SATA counterparts.
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Serial ATA (SATA)
The Serial ATA (SATA) interface specification, first introduced in August 2001, is the successor to the parallel ATA (Advanced Technology Attachment) storage interface. Compared to Parallel ATA technologies Serial ATA delivers faster throughput, great bandwidth and improved performance.
Serial ATA implements data throughput up to 3.0Gb per second and has the ability to remove or add devices while operating (hot swapping), thinner cables that let air cooling work more efficiently, and more reliable operation with tighter data integrity checks. These features and performance can represent substantial improvement over previous generations of ATA interface technology.
More information about the Serial ATA Working Group and SATA technology is available at www.serialata.org.
More Benefits of SATA interface:
- The SATA interface is stable and relatively easy to integrate.
- SATA has built in power savings command modes that can allow for reduced power usage when idle.
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Keep in mind:
- While SATA HDDs can function on a SAS interface, the reverse is not true. SAS HDDs will not function on a SATA interface.
- SATA is not backwards compatible with parallel ATA.
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Serial ATA (SATA) Hard Disk Drives: Enterprise Class
SATA hard disk drives are ideal for high capacity environments where cost per GB is a key factor. With two drive speeds, 7.2 and 5.4 RPM customers can optimize their storage requirements by using low cost SATA HDDs for less frequently accessed data or archiving purposes.
Typical attributes of drives utilizing the SATA interface are high capacity, low dollar per gigabyte and reliability ratings that are below the MTBF rating of SAS 10K and 15K RPM HDDs.
More Benefits of SATA HDDs:
- SATA drives typically draw less power than traditional SAS HDDs due to slower RPM speeds.
- SATA drives have the best dollar per gigabyte compared to SAS drives.
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Keep in mind:
- SATA HDDs are single port and not capable of being utilized in dual port environments without the addition of an interposer designed for this purpose.
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Fibre Channel - Arbitrated Loop (FC-AL)
Fibre Channel - Arbitrated Loop (FC-AL) is an exceptionally high-bandwidth industry-standard interface primarily targeted toward high-end storage and similar demanding applications. Today, FC-AL is the fastest hard disk interface with bandwidth support up to 4Gb/sec. FC-AL also offers the architecture, distance and connectivity required in many Storage applications.
FC-AL uses fiber optic cabling in a loop configuration which can produce maximum transfer speeds of up to 4Gb/second and is designed to connect up to 127 devices as far as 10 kilometers apart, enabling data storage in remote, secure locations distant from the server.
More benefits of FC-AL interface:
- FC-AL devices can be dual ported, providing two simultaneous input/output sessions that doubles maximum throughput
- FC-AL enables "hot swapping," so you can add and remove hard drives without interrupting system operation, an important option in server environments.
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Keep in mind:
- FC-AL adapters tend to cost more than SAS adapters
- FC-AL is currently the fastest interface at 4Gb but is expected to be passed in maximum bandwidth by the next generation of SAS interface at 6Gb
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Fibre Channel - Arbitrated Loop (FC-AL) Hard Disk Drives
Fibre Channel HDDs are very similar to SAS and SATA drives in terms of form factors and spin speeds.
Fibre Channel HDDs have long been the mainstay of large storage solutions because they can function at great distances away from the initiator and its ability to connect large numbers of drives to a single FC-AL environment.
More benefits of Fibre Channel HDDs:
- Fibre Channel HDDs are dual ported, providing two simultaneous input/output sessions
- Fibre Channel HDDs are hot-swap capable so users can add and remove a hard drives without interrupting system operation.
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Keep in mind:
- Fibre Channel HDDs are typically utilized in unique environments and are not compatible to be used on SAS or SATA interfaces
- Long term, SAS is projected to replace Fibre Channel HDDs within the IT industry
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