RAID Systems with CompactPCI® Serial

2009-08-24, Manfred Schmitz

An important reason for extending CompactPCI® by CompactPCI® Serial CPCI-S.0 was the possibility to realize RAID systems. For this reason SATA/SAS signals are available on the backplane beside PCI Express®, USB and Ethernet. This way, each peripheral slot can be used as a hard disk slot. As the CompactPCI® Serial standard allows eight peripheral slots, up to eight hard disks can be directly controlled by the system slot. If a PICMG 2.30 CPU (CompactPCI® PlusIO) is used, as many as four hard disks can be directly controlled.

The CompactPCI® mechanics, which has been taken over unchanged for CompactPCI® Serial, offers all characteristics which are required for a RAID shuttle:

• Robust plug-in mechanics.
• Switch in the handle to signal removal of the hard drive.
• (Blue) LED to signal that the board may be removed.

CompactPCI® Serial includes more mechanisms which are required for a RAID system. For example it is possible to determine the disk to be changed via the physical addressing. A management bus which is compliant to the SFF 8485 specification provides the necessary communication between the RAID controller and the shuttle. SFF stands for "Small Form Factor Committee", www.sffcommittee.org. Companies like Seagate, Sun, LSI, IBM and Intel® promote standards under its roof. The SFF 8485 standard defines a serial GPIO bus (SGPIO) specially in connection with Serial Attached SCSI (SAS) and Serial ATA (SATA). It serves for exchanging data between the RAID controller and the hard disk shuttle. It can transmit simple status information (e.g. "switch closed", "hard-drive powered") and control LEDs or monitor the power supply of the hard drive. The SGPIO bus is dedicated to the SATA/SAS functionality and independent of the system management bus of the CompactPCI® standard.
The SGPIO bus uses four signals for data transmission:

• SClock: A clock cycle signal driven by the RAID controller.
  
• SLoad: A synchronization signal to determine the start and the end of a transmission frame - also driven by the RAID controller.
  
• SDataOut: The output data signal of the RAID controller.
  
• SDataIn: The data input signal of the RAID controller. Here the shuttles transmit the status information while the time slot in the data frame corresponds to the physical address.

Using SATA port multipliers, even more than eight drives can be controlled in a CompactPCI® Serial system. For this, a custom backplane is needed which can have more than 8 SATA/SAS slots. A customer-specific solution could be to only specify part of the slots for SATA/SAS. Such a system could for example consist of eight slots for PCI Express®, USB and Ethernet as well as of eight additional slots for SATA/SAS hard drives.


Example of a digital video recorder for traffic supervision

Manfred Schmitz, Technical Director MEN Mikro Elektronik

German version on ElektronikPraxis: RAID-Systeme mit CompactPCI Plus

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