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Serial Attached SCSI (SAS) is becoming the new storage standard in the corporate environment. Being new, SAS brings many questions to the mind of IT Managers and others. This FAQ will attempt to answer the most common of these questions.

What is SAS (Serial Attached SCSI)?

Serial Attached SCSI (SAS) is an evolution of SCSI to a much faster serial interface. This satisfies the enterprise storage requirement of scalability, performance, reliability and manageability. It also provides an infrastructure for both SAS drives and SATA disk drives. Since the SAS interface is compatible with SATA devices, this provides users with almost unlimited choices for server and storage systems structure. SAS drives, with their outstanding speed and reliability can be used for critical online storage while SATA drives, with their higher capacity and significantly lower cost, can be used for less critical storage requirements.

Why was Serial Attached SCSI developed?

SAS was developed to solve future direct attach storage requirements. It provides compatibility with SATA and offers compatibility with SCSI as well as SCSI reliability, performance and manageability.

Aren't parallel interfaces faster than serial interfaces?

Previously, parallel interfaces were preferable to serial because their multiple data paths allowed for greater throughput than the single data path of serial interfaces. New developments in VLSI technology, however, have enabled serial interfaces to make dramatic speed increases. Serial interfaces do not have the complex timing and interference issues that hinder parallel interface development. Serial Attached SCSI features higher throughput and greater potential for advancement in the future as compared to parallel SCSI.

Is parallel SCSI now obsolete?

No. Parallel SCSI has played a fundamental role in enterprise data storage and will continue to do so. Serial Attached SCSI, however, is a strong complement to and matches the excellent reliability and robustness of parallel SCSI, while significantly expanding SCSI storage in terms of speed, scalability and flexibility. For instance, parallel development has stopped at 300GB SCSI hard drives while we already have in stock 450GB SAS drives and 1TB SAS drives are not far off.

Will migrating from parallel SCSI to SAS be difficult?

When Seagate and other companies collaborated to define Serial Attached SCSI standards, ease of migration was a primary consideration. SAS was engineered to be compatible with existing SCSI command sets thus preserving your investment in storage management and enterprise application software.

What is the difference between Serial Attached SCSI (SAS) and Serial ATA (SATA)?

Serial Attached SCSI is an enterprise storage solution that delivers the superior performance, reliability and scalability demanded in mission-critical applications. Serial ATA is primarily intended for desktop applications and suitable for use in low work load non-mission-critical environments where low cost is a high priority. This allows the development of tiered SAS and SATA storage environments where SAS is used for operating the mission-critical applications and SATA is used for providing huge amounts of storage at a low cost.

What are Small Form Factor SAS Drives?

Small form factor (SFF) 2.5 inch SAS enterprise hard drives are a new class of storage solution that feature a 70% smaller physically size and up to 40% less power consumption and heat generation while maintaining true enterprise level performance and reliability.

Because of their smaller size and lower power consumption, SFF drives have found applications in Blade Servers, storage consolidation and data center environments. SFF drives are available in capacities ranging from 36GB to 146GB with higher capacities on the horizon.

Years ago, adding a new devices to your PC was at times a nightmare. You had to manually configure resources such as interrupt levels and shared memory for each new device. Many times this created conflicts with existing devices that had to be resolved.

Along came Plug and Play (PnP) technology that changed all that and simplified adding new devices to your PC. PnP devices configured themselves by automatically adjusting their settings to work properly in your PC.

Universal Plug and Play (UPnP) does for your network what PnP did for your PC. UPnP makes adding a device to your network as simple as plugging it onto your network and turning it on.

A good example of such a device is the External 2-Bay Network Storage Enclosure DNS-323. The D-Link DNS-323 2-Bay Network Storage Enclosure, when used with internal SATA drives, enables you to share documents, files, and digital media such as music, photos, and video with everyone on the home or office network. Remotely accessing files through the Internet is also possible with the built-in FTP server2. Whether you are allowing access locally or over the Internet, keep data safe by only giving rights to specific users or groups. When configuring the DNS-323, you can create users and groups and assign them to folders with either read or read/write permissions.

How UPnP Works

When a device is first connected to a UPnP network, it must first obtain an IP address. This can be from the networks DHCP server or, if there is none, the device must self assign an IP to itself. Next, the device will advertise its presence on the network by providing a description of itself and the services it offers.

A control point receives this description which includes a list of actions related to each of the services the device is offering and information on the state of the device. The control point then send action requests to the device. The result of these requests are published on the network by event messages sent by a service on the device. If applicable, the control point will send a web page to the user's browser that will allow the user to manage the device and monitor its status.

DELL Rail Kits

DELL supplies three types of rail kits for mounting DELL servers into a rack. Dell's rail kits will work only with DELL servers - no other brand of server is supported.

RAPID RAILS

Rapid Rails, also knows as "quick connect" rails, can be used with DELL racks as well as with other standard 19" four post cabinet racks with the standard square mounting holes. Rapid Rails use square, tab-style connectors to mount in the rack.

VERSA RAILS

Versa Rails are intended to be used with non Dell manufactured 19" four post cabinets. These cabinets use threaded, round holes and screws to mount the rails in the rack. VERSA RAILS may also be used with a DELL rack but they can not be snapped in and must instead be mounted using screws.

2-POST RAILS

The 2-Post Rail kits are used with standard telco racks that have 3" or 6" posts that employ screws for mounting. These rack kits cannot be used on four post racks.

An AVS Forum User posted "IMHO, adding anything less than 300GB [to a Scientific Atlanta 8300HD] is a waste. And while one is at it, why not go for 400GB or 500GB?"

This is an excellent question. Let me start by pointing out that we are a storage specialist (primarily SCSI and SATA). We have plenty of SATA drives in stock, including the Seagate 400GB NCQ compliant 7.2K model (that one of the other users is looking for).

We have performed multi-week testing on our "8300HD approved" kits. As such, our 250GB and 160GB are GUARANTEED to be fully compatible with the 8300HD.

We have tried to find a large (over 300GB) SATA drive that works =reliably= with the 8300HD. So far we have experienced a variety of problems, the most common of which is that the drive goes off line intermittently (fixed with a cold reboot of the DVR). We had hoped to have a 400GB and 500GB bundle by now (to offer as an approved 8300HD upgrade). Our feeling is that this is an appliance, hence reliability is more important than capacity.

If you want to buy a large (400-500GB) drive from us, bundled with cable and enclosure, we will happily sell you one! (assembled or unassembled). However, we will not guarantee it to be compatible with the DVR. Its functionality will only be guaranteed with PCs.

Our conclusion is that the 8300HD's design is the limiting factor here, not some "conspiracy" by Maxtor to milk the market by only offering smaller drives.
If anyone has truly tested a large (400-500GB) capacity drive, that works (long term) we would be very excited to conduct our own testing, with a view to offering it to the market.

That said, we are offering greater value than Maxtor. Scientific Atlanta clearly document that 7.2K drives will be required for the Multi-Room feature (when activated). Maxtor Quickview Expander drives are 5.4K RPM, ours are 7.2K. Also, we've noticed that 5.4K drives have a propensity to display video glitches when recording two programs and playing a third. This is mostly eliminated by the 7.2K drives we offer. Of course, our prices are lower also, plus we offer real-time actual cost FedEx shipping rates.

Since, I'm rambling, I may as well mention the fan/fanless enclosure consideration. We too would LOVE to offer a fanless enclosure. However, our experience shows that only the most substantial (hundreds of dollars!) cases can reliably handle the heat dissipation required to maintain the longevity of drive. Over the life span of the hard drive/DVR these drives are really going to get a work-out. This type of application really generates heat, and no-one wants their drive cooked. We've tested and sourced cases with high-quality fans that make minimal noise. You can hear a light hum in a perfectly silent room. However, as soon as you turn the TV on, even at low volumes, you'd be unaware of the fan. I live with one in my home theatre and it's fine.

In conclusion, we'd really like our customers to understand that we're not just taking any external SATA drive and pretending it's for the 8300HD. We're actually investing time and money to find solutions that work! We'd love to make them better and bigger, and look forward to your input.

This article addresses INTERNAL SCSI device interfaces only. Please see our articles on external interfaces if you are looking for an external device. There are four internal interface types: 80pin, 68pin, 50pin and 25pin.

We discuss 80-pin and 68-pin here, as they are most relevant today. If your computer was built after 1998 it is highly unlikely that you would need to consider 50pin (used by Narrow/Fast SCSI II devices) or 25pin SCSI (used by SCSI I devices). Please see our other article for details on these interfaces.

The 80pin SCSI interface uses the SCA-2 connector and are ONLY found on hot-swappable SCSI hard drives. There is in fact no 80pin SCSI standard, these are 68pin SCSI devices (16 bit) with 12pins that provide integrated power for hot-swappability (A.K.A. hot-pluggable). 80 pin hot-swappable SCSI hard drives are ONLY used in chassis (usually servers) that have a hot-swap backplane designed for SCSI hard drives with an SCA-2 connector. Good examples of these servers are Dell PowerEdge servers, IBM eServer xSeries servers, Compaq Proliant servers, IBM Netfinity Servers, HP Proliant servers, HP 9000 servers, Sun Enterprise server, Sun Netra servers, etc.You do NOT connect a cable to these drives, they slide into the front of the server case, and clip into place (hence hot-pluggable). This connector is trapezoidally shaped and is 2.25" or 57mm long (longest side of the trapezium) and 0.25" or 6mm wide.

N.B. Servers that use hot-swappable drives, still use 68pin SCSI controllers. The hot-swap backplane has a 68pin connector on it to connect to the controller. The power integration is done by the backplane. Therefore there 80pin controllers, cables or terminators do NOT exist!

The 68pin SCSI interface (16 bit) is the current (1995-2006+) inteface type and is found on ALL current model SCSI controllers, including SCSI RAID controllers. 68pin devices have a separate power connector; usually the standard 4 prong Molex connector found inside computers. Therefore to install a 68pin device you must attach a SCSI cable and a power cable. All current generation , internal SCSI tape drives, internal SCSI CD-ROM drives and internal SCSI DVD-ROM drives use 68 pin SCSI connectors. Most current generation internal SCSI hard drives on desktop computers and workstations use 68pin SCSI hard drives. Good examples of these systems are Dell Precision workstations, Dell Optiplex PCs, Compaq Prosignia PCs, HP workstations, HP AlphaStation workstations, Apple G3 300 minitower, Apple G4 workstations, Apple G5 workstations, some AVID system, SGI Tezro workstations, SGI Fuel workstations & Sun Ultra Workstations. This connector is trapezoidally shaped and is 1.81" or 45mm long (longest side of the trapezium) and 0.2" or 5mm wide.

DiscounTechnology.com - SCSI Hard Drive Specialists