Page 1:SAS Raises Storage Capabilities To Higher Power
Page 2:SCSI's Details And History
Page 3:From SATA...
Page 6:SAS Backplanes & Cables
Page 7:SAS As Part Of SAN Storage Solutions
Page 8:2.5" SAS Hard Drives
Page 9:Fujitsu MAY2073RC
Page 10:3.5" SAS Hard Drives
Page 11:Hitachi UltraStar 15K147 SAS (HUS151414VLS300)
Page 12:Maxtor Atlas 15KII
Page 13:Seagate Cheetah 15K.4B 147 GB (ST3146854SS)
Page 14:Host Adapters
Page 15:Adaptec SAS 48300
Page 16:RAID Adapters
Page 17:LSI Logic SAS3442X
Page 18:SAS Appliances And Enclosures
Page 19:Storage System
Page 20:Adaptec SANbloc S50 JBOD, Continued
Page 21:Test Setup
Page 22:Hard Drive Test Results
Page 23:3.5" SAS Data Transfer Diagrams
Page 24:Data Transfer Performance
Page 25:Access Time
Page 26:I/O Performance
Page 27:SAS RAID Test Results (4 And 10 Drive Arrays)
SCSI's Details And History
First of all, we believe it is important to understand SAS' history. SCSI, the small computer system interface, has always served as a professional bus for attaching storage and certain other device types to computers. Workstation and server class hard drives were and still are based on SCSI technology. In contrast to mainstream ATA with only two drives per controller port, SCSI allows for daisy-chaining up to 15 devices and it comes with a powerful command protocol. Devices require a unique SCSI ID, which is either set manually on each device, or it can be assigned via SCAM (SCSI Configuration Automatically). Since IDs are not necessarily unique across two or more SCSI adapters, so-called LUNs (Logical Unit Numbers) are put together to identify units across complex SCSI environments.
SCSI hardware is more flexible and robust than ATA (also called IDE, which stands for Integrated Drive Electronics). Devices can be attached internally or externally and use cables at lengths of up to 39.37 ft (12 m) () as long as the bus is terminated properly, which eliminates signal reflection. As SCSI evolved, multiple standards became available, covering different bus widths, clock speeds, connectors and voltage signalling (Fast, Wide, Ultra, Ultra Wide, Ultra2, Ultra2 Wide, Ultra3, Ultra320 SCSI). The good part is they all share a common command set.
All SCSI communication takes place between an initiator (host adapter) that sends commands, and a target drive that responds. Immediately after receiving a command sequence, the target returns a so-called sense code (busy, error or success check condition), so the initiator knows whether or not it will get the response it wants.
SCSI has almost 60 different commands. These are classified into four categories: non-data, bi-directional, read and write data commands.
The limitations of SCSI become obvious as you keep adding drives to the bus. While few hard drives do not saturate the bandwidth of Ultra320 SCSI (320 MB/s), five or more drives easily do. Adding a second host adapter to balance the load is a solution, but it adds cost. In addition, cramming multiple drives into a server system is neither elegant nor advisable. Cables are also an issue; the twisted 80-pin models are very expensive. If you want drives to be hot-swappable, e.g. for easy replacement of defective drives, backplanes are required for proper removal and insertion of devices into the SCSI chain.
Ideally, your drives should be placed into separate storage enclosures or appliances that usually include all the hot-swapping capabilities as well as managing features. Altogether there are lots of professional SCSI-based solutions available. But all of these come at a price premium, which is why SATA has had tremendous impetus over the last couple of years. Though SATA will never be adequate for high-end enterprise requirements, it perfectly complements SAS when it comes to creating new, scalable solutions for next-generation networked environments.
SAS does not share one bus for multiple devices. Source: Adaptec.
- SAS Raises Storage Capabilities To Higher Power
- SCSI's Details And History
- From SATA...
- SAS Backplanes & Cables
- SAS As Part Of SAN Storage Solutions
- 2.5" SAS Hard Drives
- Fujitsu MAY2073RC
- 3.5" SAS Hard Drives
- Hitachi UltraStar 15K147 SAS (HUS151414VLS300)
- Maxtor Atlas 15KII
- Seagate Cheetah 15K.4B 147 GB (ST3146854SS)
- Host Adapters
- Adaptec SAS 48300
- RAID Adapters
- LSI Logic SAS3442X
- SAS Appliances And Enclosures
- Storage System
- Adaptec SANbloc S50 JBOD, Continued
- Test Setup
- Hard Drive Test Results
- 3.5" SAS Data Transfer Diagrams
- Data Transfer Performance
- Access Time
- I/O Performance
- SAS RAID Test Results (4 And 10 Drive Arrays)