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- Mar 13, 2002
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SCSI 101
I hadn't seen one of there yet here. So...
SCSI is small computer systems interface and the specification describes the interface in the same way that the ATA specifications describe the behavior of the IDE interface.
Interfaces:
SCSI interfaces come in several varieties grouped by their maximum theoretical bandwidth and width of the bus. Interfaces include:
SCSI-1 5MB/Sec 8 bit SCSI Bus
SCSI-2 5MB/Sec 8 bit SCSI Bus
SCSI-2 Fast 10MB/Sec 8 bit SCSI Bus
SCSI-2 Fast Wide 20MB/Sec 16 bit SCSI Bus
SCSI Ultra 20MB/Sec 8 bit SCSI Bus
SCSI Ultra Wide 40MB/Sec 16 bit SCSI Bus
Ultra2 Wide 80MB/sec 16 bit SCSI Bus
Ultra160 Wide 160MB/sec 16 bit SCSI Bus
Ultra320 Wide 320MB/sec 16 bit SCSI Bus
With the last three being the most commonly used interfaces. Anything slower is usually not worth the time, expense and effort.
Unlike DMA modes for the ATA specification (with the exception of PIO mode devices), connection of compatible lower speed devices can cause the interface to lower its performance to the level of the slowest device. When all Wide devices are used, the bus will use the lower speed protocol while accessing the slower devices. Use of narrow SCSI-2 devices on a Ultra2 Low Voltage Differential (LVD) port will cause the entire bus to run at the SCSI Ultra 20MB/sec level or slower and mixing theses drives should be avoided at all costs. Very few people still use the narrow bus (8 bit), but devices are still available, most commonly in optical drives.
The number of devices, which can be attached to any channel, varies with the type of controller used and usually the cabling requirements. Narrow (8 bit) controllers can support up to 7 devices and the controller. Wide (16 bit) controllers can handle 15 devices and the controller. The reason that this is noted per channel is due to the many multi channel controllers in use. Controllers can commonly come with 4 or more channels, particularly in higher end RAID controllers.
The devices on a SCSI chain use identification numbers rather then the master/slave configuration of IDE specifications. The devices are numbered from 0 to 7 on narrow interfaces and 0 to 15 on wide interfaces, with ID 7 typically being the controller on both bus styles. For a drive to be bootable, it needs to be ID 0 on most controllers. ID also can determine the priority of the devices with the lower numbers(0-6) having increased priority.
Connections:
Over the years, SCSI has used a variety of connection styles. This has added some confusion, since there are adapters for the various types of internal and external cabling. Currently, the cabling that you are most likely to see is the 68 pin internal type. It supports protocols ranging up to U320 and is a standard connection interface which has adapters available to both 80 pin SCA and old 50 pin narrow connectors. The 80 pin SCA connectors have provide both power and addressing functions to the SCA drives(80 to 68 pin adapters are available if you have these drives). The 68 pin type connectors rely on jumpers on the drive to set ID and other functions and have separate power connections. The 50 pin connectors are similar to IDE connectors and are generally found on very old HDDs or optical drives up to present releases. External connections are completely different from internal connectors and the most common types are high density and very high density(VHDCI) connectors.
Termination:
All verities of SCSI require termination of cabling, much like old 10baseT cables. Old 50 pin devices often were able to self terminate the chain, but newer devices(all 68 or 80 pin) use separate terminators. These can be either active or passive and will be attached at the opposite end of the chain from the controller or host bus adapter(HBA). Cabling should be arranged as seen below:
HBA-----Drive-----Drive-----Drive-----Terminator
RAID:
RAID is available with some SCSI controllers, typically, only on the more expensive varieties. Software RAID can easily be implemented with any controller, but you’ll be limited to non-Boot drives for arrays. If you want to boot from the array, a SCSI RAID controller will be necessary. Unlike most IDE RAID implementations, SCSI RAID controllers normally have onboard cache and processor to perform the RAID calculations. This enables a far wider assortment of array configurations, including many that are proprietary with controllers. The limitations inherent in this include: expense, low processor speed on the controller(software RAID can be faster), often larger or full length cards, limited cache quantities, increase in seek and access times and often limited SCSI channels(which can become saturated). There are benefits that often offset these things: increased data redundancy with levels above 0, low to no processor loading for RAID processes, many options including redundant controllers with failover, when using SCA drives on a backplane hot swap drives and higher level controllers have restorable configuration information on the drives increasing survivability in the event of a controller needing to be replaced.
Benefits:
The reasons for utilizing SCSI include data security, speed, device compatibility, high number of available devices per channel, flexible configuration options, high bandwidth controllers and wide variety of supported RAID levels. Data security is a standard implementation with most SCSI controllers. By default, they do CRC checking on data written to the drives and have bad sector scanning enabled by default. This reduces the number of soft errors that occur on the drives. PATA IDE does not have this capacity, I’ve heard rumors that SATA is supposed to support it in either SATA-1 or SATA-2, but have not confirmed it as of yet. Disk speed in terms of STR, access time and burst rates can be higher with SCSI disks than any media other than solid state. Wide varieties of equipment are supported in SCSI formats, though with the advances in optical drives, it’s usually better to leave them attached with IDE. With any of the Wide SCSI options up to 15 devices can be supported per channel and multi channel controllers are widely available. SCSI controllers are available with up to PCI-X 64/133 interfaces. Finally, hardware RAID controllers can be found that support the widest range of RAID levels.
Drawbacks:
The most noticeable drawback to SCSI is cost. Integrated IDE requires a cable and drive. SCSI typically requires a controller, cable, drive and terminator. With high speed disks, the drives are expensive and low capacity. SCSI is initially harder to understand than IDE and can create a great deal of frustration in first time users. Finally, as with other interfaces, advertising is better than reality. Just because the interface is 160MB/s or faster, doesn’t mean that’s what you’ll get. The drives are mechanically limited in their available speed and to go faster always costs more. SCSI hardware RAID can be faster, but often is limited by the processor on the controller. Software RAID can actually be faster than hardware RAID due to the difference in processor speed. Finally, SCSI is normally optimized for high IO rates and multiple user configurations. This may be less advantageous to single users and satisfactory performance can sometimes be obtained, with much less expense, using IDE interface drives.
How to order:
Make sure that you have everything together that you’ll need. At a minimum, you’ll need a controller, drive, cable and terminator. You can use faster cables and terminators with slower drives and controllers without any performance problems. Slower cables and terminators will limit performance normally. If you’re planning to upgrade later, go ahead and get the best cables and terminators that you can afford. If you are using SCA drives you will need a SCA adapter to go from 80 pin SCA to 68 pin SCSI. This will have a molex power connection and jumpers to set drive settings. You’ll need one adapter per drive. You need one terminator per cable and it goes at the end furthers from the controller. If you are running on a non workstation or server mainboard with a regular 32 bit 33Mhz PCI bus, you will not need more than a U160 controller, unless you are planning to migrate to a faster bus later. The PCI bus bandwidth is not sufficient to require more. One important note on ordering cables, the controller may be considered as a device. When you see a cable advertised as 2 device, it sometimes has three positions, one for the controller, drive and terminator. Check with the vendor to make sure. Make sure that you order cables with enough positions for the drives you intend to use, preferably with an extra connection or more for future expansion
I hadn't seen one of there yet here. So...
SCSI is small computer systems interface and the specification describes the interface in the same way that the ATA specifications describe the behavior of the IDE interface.
Interfaces:
SCSI interfaces come in several varieties grouped by their maximum theoretical bandwidth and width of the bus. Interfaces include:
SCSI-1 5MB/Sec 8 bit SCSI Bus
SCSI-2 5MB/Sec 8 bit SCSI Bus
SCSI-2 Fast 10MB/Sec 8 bit SCSI Bus
SCSI-2 Fast Wide 20MB/Sec 16 bit SCSI Bus
SCSI Ultra 20MB/Sec 8 bit SCSI Bus
SCSI Ultra Wide 40MB/Sec 16 bit SCSI Bus
Ultra2 Wide 80MB/sec 16 bit SCSI Bus
Ultra160 Wide 160MB/sec 16 bit SCSI Bus
Ultra320 Wide 320MB/sec 16 bit SCSI Bus
With the last three being the most commonly used interfaces. Anything slower is usually not worth the time, expense and effort.
Unlike DMA modes for the ATA specification (with the exception of PIO mode devices), connection of compatible lower speed devices can cause the interface to lower its performance to the level of the slowest device. When all Wide devices are used, the bus will use the lower speed protocol while accessing the slower devices. Use of narrow SCSI-2 devices on a Ultra2 Low Voltage Differential (LVD) port will cause the entire bus to run at the SCSI Ultra 20MB/sec level or slower and mixing theses drives should be avoided at all costs. Very few people still use the narrow bus (8 bit), but devices are still available, most commonly in optical drives.
The number of devices, which can be attached to any channel, varies with the type of controller used and usually the cabling requirements. Narrow (8 bit) controllers can support up to 7 devices and the controller. Wide (16 bit) controllers can handle 15 devices and the controller. The reason that this is noted per channel is due to the many multi channel controllers in use. Controllers can commonly come with 4 or more channels, particularly in higher end RAID controllers.
The devices on a SCSI chain use identification numbers rather then the master/slave configuration of IDE specifications. The devices are numbered from 0 to 7 on narrow interfaces and 0 to 15 on wide interfaces, with ID 7 typically being the controller on both bus styles. For a drive to be bootable, it needs to be ID 0 on most controllers. ID also can determine the priority of the devices with the lower numbers(0-6) having increased priority.
Connections:
Over the years, SCSI has used a variety of connection styles. This has added some confusion, since there are adapters for the various types of internal and external cabling. Currently, the cabling that you are most likely to see is the 68 pin internal type. It supports protocols ranging up to U320 and is a standard connection interface which has adapters available to both 80 pin SCA and old 50 pin narrow connectors. The 80 pin SCA connectors have provide both power and addressing functions to the SCA drives(80 to 68 pin adapters are available if you have these drives). The 68 pin type connectors rely on jumpers on the drive to set ID and other functions and have separate power connections. The 50 pin connectors are similar to IDE connectors and are generally found on very old HDDs or optical drives up to present releases. External connections are completely different from internal connectors and the most common types are high density and very high density(VHDCI) connectors.
Termination:
All verities of SCSI require termination of cabling, much like old 10baseT cables. Old 50 pin devices often were able to self terminate the chain, but newer devices(all 68 or 80 pin) use separate terminators. These can be either active or passive and will be attached at the opposite end of the chain from the controller or host bus adapter(HBA). Cabling should be arranged as seen below:
HBA-----Drive-----Drive-----Drive-----Terminator
RAID:
RAID is available with some SCSI controllers, typically, only on the more expensive varieties. Software RAID can easily be implemented with any controller, but you’ll be limited to non-Boot drives for arrays. If you want to boot from the array, a SCSI RAID controller will be necessary. Unlike most IDE RAID implementations, SCSI RAID controllers normally have onboard cache and processor to perform the RAID calculations. This enables a far wider assortment of array configurations, including many that are proprietary with controllers. The limitations inherent in this include: expense, low processor speed on the controller(software RAID can be faster), often larger or full length cards, limited cache quantities, increase in seek and access times and often limited SCSI channels(which can become saturated). There are benefits that often offset these things: increased data redundancy with levels above 0, low to no processor loading for RAID processes, many options including redundant controllers with failover, when using SCA drives on a backplane hot swap drives and higher level controllers have restorable configuration information on the drives increasing survivability in the event of a controller needing to be replaced.
Benefits:
The reasons for utilizing SCSI include data security, speed, device compatibility, high number of available devices per channel, flexible configuration options, high bandwidth controllers and wide variety of supported RAID levels. Data security is a standard implementation with most SCSI controllers. By default, they do CRC checking on data written to the drives and have bad sector scanning enabled by default. This reduces the number of soft errors that occur on the drives. PATA IDE does not have this capacity, I’ve heard rumors that SATA is supposed to support it in either SATA-1 or SATA-2, but have not confirmed it as of yet. Disk speed in terms of STR, access time and burst rates can be higher with SCSI disks than any media other than solid state. Wide varieties of equipment are supported in SCSI formats, though with the advances in optical drives, it’s usually better to leave them attached with IDE. With any of the Wide SCSI options up to 15 devices can be supported per channel and multi channel controllers are widely available. SCSI controllers are available with up to PCI-X 64/133 interfaces. Finally, hardware RAID controllers can be found that support the widest range of RAID levels.
Drawbacks:
The most noticeable drawback to SCSI is cost. Integrated IDE requires a cable and drive. SCSI typically requires a controller, cable, drive and terminator. With high speed disks, the drives are expensive and low capacity. SCSI is initially harder to understand than IDE and can create a great deal of frustration in first time users. Finally, as with other interfaces, advertising is better than reality. Just because the interface is 160MB/s or faster, doesn’t mean that’s what you’ll get. The drives are mechanically limited in their available speed and to go faster always costs more. SCSI hardware RAID can be faster, but often is limited by the processor on the controller. Software RAID can actually be faster than hardware RAID due to the difference in processor speed. Finally, SCSI is normally optimized for high IO rates and multiple user configurations. This may be less advantageous to single users and satisfactory performance can sometimes be obtained, with much less expense, using IDE interface drives.
How to order:
Make sure that you have everything together that you’ll need. At a minimum, you’ll need a controller, drive, cable and terminator. You can use faster cables and terminators with slower drives and controllers without any performance problems. Slower cables and terminators will limit performance normally. If you’re planning to upgrade later, go ahead and get the best cables and terminators that you can afford. If you are using SCA drives you will need a SCA adapter to go from 80 pin SCA to 68 pin SCSI. This will have a molex power connection and jumpers to set drive settings. You’ll need one adapter per drive. You need one terminator per cable and it goes at the end furthers from the controller. If you are running on a non workstation or server mainboard with a regular 32 bit 33Mhz PCI bus, you will not need more than a U160 controller, unless you are planning to migrate to a faster bus later. The PCI bus bandwidth is not sufficient to require more. One important note on ordering cables, the controller may be considered as a device. When you see a cable advertised as 2 device, it sometimes has three positions, one for the controller, drive and terminator. Check with the vendor to make sure. Make sure that you order cables with enough positions for the drives you intend to use, preferably with an extra connection or more for future expansion
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