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[ In Focus ] |
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Alphabet soup
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by Wayne Cole
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Media storage questions used to be simply a matter of which version of SCSI was required. Now, with media access by multiple clients over networks in addition to "desk side" storage and newer connectivity standards, determining what type of storage you need can be confusing. Here's a quick guide through the current acronyms in the hopes that it will provide you with a clearer storage picture.
SAS -- Serial Attached SCSI
SAS was designated by the SCSI Trade Association as a replacement for SCSI for direct attached storage (DAS). It uses the less expensive SAS and SATA drives with a serial connection that's faster than parallel SCSI connections supporting data rates from 1.5 Gbps to 6 Gbps. But SAS still uses the SCSI command set to talk to its attached device.
Yet, SAS does have some differences from SCSI. While SCSI provided a multi-drop target capability, SAS is a simple point-to-point connection. The endpoint can be either a single hard disk drive or a disk array. So, while you could connect up to 31 devices to later SCSI controllers, with a SAS controller you must add a SAS expander to connect multiple targets to a single SAS initiator.
Because each SAS device comes out of the factory with a unique World Wide Name or WWN (similar to an Ethernet device's MAC address), users can build a SAS domain of up to 16,256 devices controlled by a single initiator through a slew of edge expanders. The SAS bus speed applies across the entire domain, whereas a SCSI's bus speed is shared by all the devices on the chain. Also, SAS speed can be increased by adding multiple pathways or "lanes" between the initiator and target device.
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| A mini-SAS connector is required to support planned bus speeds of 12 Gbps. |
The confusing termination issues associated with SCSI don't affect the point-to-point SAS. In addition, SAS allows cable runs of up to 26 feet, more than twice most SCSI capabilities.
Specifying SAS connectors requires some forethought because they vary depending on whether the connection is to external devices, internal devices, or SATA drives. While there are not yet distinct levels of SAS as there were with SCSI, any plan to support the proposed 12 Gbps speed might call for the use of mini-SAS connectors (also known as Molex iPASS connectors) today.
SATA -- Serial ATA
Like SCSI, the Advanced Technology Attachment (ATA) originally used a parallel bus. By moving to a serial interface, speed increases similar to SAS were possible using less expensive SATA devices.
Designed as a competitor to USB and FireWire storage on consumer PCs, SATA initially specified a speed of 1.5 Mbs. The later revised 3 Gbps SATA bus obtained the misnomer "SATA 2," which was actually planned for future higher speed SATA connections.
It's also a point-to-point scheme where a single controller requires a "port multiplier" to communicate with multiple devices. With SATA, however, the bandwidth is shared by the connected devices.
The Advanced Host Controller Interface (AHCI) for SATA devices implements features like hot-swapping and the ability to handle multiple requests (NCQ or native command queuing). Without these features enabled, the AHCI acts like an ATA/IDE controller.
To connect external devices, you need an eSATA cable. And to access the higher speeds of SATA, unique drivers and advanced firmware settings may be needed, depending on the operating system and vintage of the host computer.
While SATA devices can be used with SAS controllers, the reverse is not true. SATA also requires a different power connection than the standard four-pin Molex. While adapters do exist, their use usually eliminates the hot-swap capability. SATA data connectors have come under fire for their fragility, and if broken at the device end, can cause the stored data to be lost.
FC -- Fibre Channel
Appearing in the mid-1990s, FC was designed for networked storage with speeds up to 12 Gbps, which allowed it to support video media production in distributed workflow environment. To support multiple simultaneous users, FC required more complex topologies than the simple point-to-point approach of SAS and SATA. That said, it was often configured using FC-P2P (Fibre Channel Point-to-Point) for single ended use.
FC-AL (Arbitrated Loop) can configure up to 127 devices in a ring topology. If one device fails, however, the entire loop will go down until the failed device is bypassed. FC hubs can be used as intelligent "loop monitors" that will automate that process.
FC-SW, or switched fabric, allows connection to over 16 million devices. As the name suggests, both individual devices and loops can all be attached to switches, creating a web-like configuration of data paths between devices and clients.
Physically, FC can be implemented with copper twisted pair or optical fiber allowing for very long cable runs compared to SAS and SATA. This makes it ideal for large campus operations with central data storage needs. Use of optical cable also lowers security risks where spying via EMF monitoring is a concern.
iSCSI, NAS, and SAN
iSCSI provides access to network attached storage (NAS) or other devices that respond to the SCSI command set. The "i" indicates that these commands are routed over a standard Ethernet connection using TCP/IP. To users, it appears the SCSI device is attached to their local computers.
Because it requires no cabling beyond an existing Ethernet, iSCSI has sometimes been called the poor man's FC. Combined with mirrored arrays, iSCSI handles data protection as well as consolidation for ease of management.
iSCSI is the most popular protocol for implementing SANs (Storage Area Networks). In a SAN, the hardware initiator may be replaced with a software driver that appears to applications like a local SCSI controller. The driver uses the network interface card to access remote storage without applications or the local file systems being any the wiser. However, where there are heavy loads, a hardware iSCSI initiator (combo network card and SCSI host bus adaptor) may improve overall network performance.
Know Your Workflow
Which setup is right for you? SAS is best for single seat editing systems and could be assembled by most power users. A distributed work system indicates a need for Fibre Channel or iSCSI. Such NAS/SAN solutions should be considered only with the recommendation of an IT professional after analysis of your operational needs and data flow.
Avastor adds HDX-1500
Avastor introduced the HDX-1500 SATA Quad interface drive last month at NAB. It provides connectivity for USB 2.0, FireWire 400 and 800, and eSATA ports in a single unit. Tailored for NLE systems, the single disk external storage solution offers data transfer rates exceeding 90 MBps. In addition, the HDX-1500 Quad supports the latest in eSATAII drive technology with drive capacities from 160 GB up to 1 TB and includes the Digidesign Pro Tools compliant Oxford 924 FireWire chipset. Internal power supply and cooling fan are standard features.
avastor.com
Wayne M. Cole, CCV, CLVI, and member of the AGCV Board of Advisors, is also the owner of IHP, a video production company located Santa Barbara, CA. Contact him at wcole@ihpweb.com.
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