Re: The SX4 challenge

[Mikael Pettersson <mikpe@it.uu.se>]

Jeff Garzik writes:
 > 
 > Promise just gave permission to post the docs for their PDC20621 (i.e. 
 > SX4) hardware:
 > http://gkernel.sourceforge.net/specs/promise/pdc20621-pguide-1.2.pdf.bz2
 > 
 > joining the existing PDC20621 DIMM and PLL docs:
 > http://gkernel.sourceforge.net/specs/promise/pdc20621-pguide-dimm-1.6.pdf.bz2
 > http://gkernel.sourceforge.net/specs/promise/pdc20621-pguide-pll-ata-timing-1.2.pdf.bz2
 > 
 > 
 > So, the SX4 is now open.  Yay :)  I am hoping to talk Mikael into 
 > becoming the sata_sx4 maintainer, and finally integrating my 'new-eh' 
 > conversion in libata-dev.git.

The best solution would be if some storage driver person would
take on the SX4 challenge and work towards integrating the SX4
into Linux' RAID framework.

If no-one steps forward I'll take over Jeff's SX4 card and just
maintain sata_sx4 as a plain non-RAID driver. Unfortunately I
don't have the time needed to turn it into a decent RAID or
RAID-offload driver myself.

/Mikael

 > 
 > But now is a good time to remind people how lame the sata_sx4 driver 
 > software really is -- and I should know, I wrote it.
 > 
 > The SX4 hardware, simplified, is three pieces:  XOR engine (for raid5), 
 > host<->board memcpy engine, and several ATA engines (and some helpful 
 > transaction sequencing features).  Data for each WRITE command is first 
 > copied to the board RAM, then the ATA engines DMA to/from the board RAM. 
 >   Data for each READ command is copied to board RAM via the ATA engines, 
 > then DMA'd across PCI to your host memory.
 > 
 > Therefore, while it is not hardware RAID, the SX4 provides all the 
 > pieces necessary to offload RAID1 and RAID5, and handle other RAID 
 > levels optimally.  RAID1 and 5 copies can be offloaded (provided all 
 > copies go to SX4-attached devices of course).  RAID5 XOR gen and 
 > checking can be offloaded, allowing the OS to see a single request, 
 > while the hardware processes a sequence of low-level requests sent in a 
 > batch.
 > 
 > This hardware presents an interesting challenge:  it does not really fit 
 > into software RAID (i.e. no RAID) /or/ hardware RAID categories.  The 
 > sata_sx4 driver presents the no-RAID configuration, while is terribly 
 > inefficient:
 > 
 > 	WRITE:
 > 		submit host DMA (copy to board)
 > 		host DMA completion via interrupt
 > 		submit ATA command
 > 		ATA command completion via interrupt
 > 	READ:
 > 		submit ATA command
 > 		ATA command completion via interrupt
 > 		submit host DMA (copy from board)
 > 		host DMA completion via interrupt
 > 
 > Thus, the "SX4 challenge" is a challenge to developers to figure out the 
 > most optimal configuration for this hardware, given the existing MD and 
 > DM work going on.
 > 
 > Now, it must be noted that the SX4 is not current-gen technology.  Most 
 > vendors have moved towards an "IOP" model, where the hw vendor puts most 
 > of their hard work into an ARM/MIPS firmware, running on an embedded 
 > chip specially tuned for storage purposes.  (ref "hptiop" and "stex" 
 > drivers, very very small SCSI drivers)
 > 
 > I know Dan Williams @ Intel is working on very similar issues on the IOP 
 > -- async memcpy, XOR offload, etc. -- and I am hoping that, due to that 
 > current work, some of the good ideas can be reused with the SX4.
 > 
 > Anyway...  it's open, it's interesting, even if it's not current-gen 
 > tech anymore.  You can probably find them on Ebay or in an 
 > out-of-the-way computer shop somewhere.
 > 
 > 	Jeff