Messages in this thread | | | Date | Wed, 13 Sep 2006 00:05:36 -0400 | From | Jeff Garzik <> | Subject | Re: [PATCH 00/19] Hardware Accelerated MD RAID5: Introduction |
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Dan Williams wrote: > On 9/11/06, Jeff Garzik <jeff@garzik.org> wrote: >> Dan Williams wrote: >> > This is a frequently asked question, Alan Cox had the same one at OLS. >> > The answer is "probably." The only complication I currently see is >> > where/how the stripe cache is maintained. With the IOPs its easy >> > because the DMA engines operate directly on kernel memory. With the >> > Promise card I believe they have memory on the card and it's not clear >> > to me if the XOR engines on the card can deal with host memory. Also, >> > MD would need to be modified to handle a stripe cache located on a >> > device, or somehow synchronize its local cache with card in a manner >> > that is still able to beat software only MD. >> >> sata_sx4 operates through [standard PC] memory on the card, and you use >> a DMA engine to copy memory to/from the card. >> >> [select chipsets supported by] sata_promise operates directly on host >> memory. >> >> So, while sata_sx4 is farther away from your direct-host-memory model, >> it also has much more potential for RAID acceleration: ideally, RAID1 >> just copies data to the card once, then copies the data to multiple >> drives from there. Similarly with RAID5, you can eliminate copies and >> offload XOR, presuming the drives are all connected to the same card. > In the sata_promise case its straight forward, all that is needed is > dmaengine drivers for the xor and memcpy engines. This would be > similar to the current I/OAT model where dma resources are provided by > a PCI function. The sata_sx4 case would need a different flavor of > the dma_do_raid5_block_ops routine, one that understands where the > cache is located. MD would also need the capability to bypass the > block layer since the data will have already been transferred to the > card by a stripe cache operation > > The RAID1 case give me pause because it seems any work along these > lines requires that the implementation work for both MD and DM, which > then eventually leads to being tasked with merging the two.
RAID5 has similar properties. If all devices in a RAID5 array are attached to a single SX4 card, then a high level write to the RAID5 array is passed directly to the card, which then performs XOR, striping, etc.
Jeff
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