[CentOS] PCI-X/PCIe RAID controller.

Sun Apr 10 10:37:45 UTC 2011
John R Pierce <pierce at hogranch.com>

On 04/10/11 3:09 AM, Christopher Chan wrote:
> OH, is that how those babies work? The backplane is not using a
> controller chip that mutiplexes the stuff? Oops, sorry.

The backplane typically has a SAF-TE 
(SCSI Accessed Fault-Tolerant Enclosure) chip which provides backplane 
management services, like notifying the raid controller or host software 
when devices are inserted or removed, as well as controlling device 
power, monitor enclosure environment, etc.    In fact, SAF-TE itself is 
a SCSI device on the same backplane.

Now, SAS backplanes do have multiplexers.  One of the big functional[*] 
differences between SAS and SATA is that SATA only supports a simple 
expander where 1 SATA channel can be expanded to several devices, 
whereas a SAS multiplexor allows N SAS channels to talk to M devices.  N 
is often 4, and M is usually the size of the drive tray.  With the SATA 
version, if you had 4 SATA channels each exanded to 4 drives for 16 
drives total, drives 0-3 would always talk to SATA channel 0, drives 4-7 
to SATA 1, etc.   with SAS, any channel can talk to any device, so you 
are less likely to run into bottlenecks

[*] For the purpose of that description, I am ignoring the big 
implementation difference in that SAS uses SCSI commands to talk to 
drives, while SATA uses IDE/ATA derived commands.   Another significant 
functional difference, SAS drives are commonly dual ported, so they can 
be connected to redundant host bus adapters or raid controllers, so if a 
controller fails, all devices are still addressable.     SAS controllers 
are designed so they can talk to SATA drives, but SATA controllers can't 
talk to SAS devices.