On 6/6/2012 11:09 AM, Dan Williams wrote: > Hardware raid ultimately does the same shuffling, outside of nvram an > advantage it has is that parity data does not traverse the bus... Are you referring to the host data bus(s)? I.e. HT/QPI and PCIe? With a 24 disk array, a full stripe write is only 1/12th parity data, less than 10%. And the buses (point to point actually) of 24 drive caliber systems will usually start at one way B/W of 4GB/s for PCIe 2.0 x8 and with one way B/W from the PCIe controller to the CPU starting at 10.4GB/s for AMD HT 3.0 systems. PCIe x8 is plenty to handle a 24 drive md RAID 6, using 7.2K SATA drives anyway. What is a bigger issue, and may actually be what you were referring to, is read-modify-write B/W, which will incur a full stripe read and write. For RMW heavy workloads, this is significant. HBA RAID does have a big advantage here, compared to one's md array possessing the aggregate performance to saturate the PCIe bus. -- Stan -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
