Re: Linux SATA RAID FAQ

[Jeff Garzik <jgarzik@pobox.com>]

Alan Cox wrote:
> end devices (eg aacraid sata boards). There are also some low end
> devices with part of the raid logic in hardware (some promise) although
> I don't believe we use that to the full yet.

Nope.  My SATA RAID FAQ mentions the Promise "RAID accelerator" stuff. 
In particular, the Promise SX4 is in fact likely to be faster with the 
_Promise_ RAID driver, rather than the Linux non-RAID driver.

I really like the SX4, and wish Linux could make better use of it...

The SX4 has an on-board DIMM (128M - 2G), through which all data _must_ 
pass.  The data transfer between host and on-board DIMM is a separate 
DMA engine and separate interrupt event from the four ATA DMA engines 
(one per SATA port).  There are several possibilities that are worth 
exploring on this card:

* Caching
* Eliminate PCI bus traffic by sending RAID1/5 writes a _single_ time to 
the card, and then multiplex to multiple attached drives from there
* Offload RAID5 XOR calculations, which becomes quite useful in 
combination with these other features
* Execute RAID1/5 resyncs and parity checks completely on the card

I have full docs and knowledge, but no idea how to hook this stuff into 
Linux md efficiently.

	Jeff