On Saturday 22 November 2003 18:07, AndyLiebman@aol.com wrote: > I want to congratulate a lot of Linux Software Raid folks. Really. I just > set up a RAID 5 array on my Linux Machine (P4-3.06 Ghz -- Mandrake 9.2) > using 5 External Firewire Drives. > > The performance is SO GOOD that I am able to write uncompressed 8-bit video > files to my array through a Copper Gigabit network! That's a sustained 18 > MB/sec -- going for 20 minutes straight. > > The first one was set up with 6 Firewire Drives that are bigger (200 GB > versus 120 GB) and that have larger onboard cache (8 MB versus 2 MB). I set > up those 6 drives as a RAID 10 array -- 3 mirrored pairs with a RAID 0 > stripe on top of that. The performance I was able to achieve with the RAID > 10 array was actually NO BETTER than what I am getting with RAID 5. Does > that make sense? Hmmm, here are some of my thoughts, maybe some of my assumptions are wrong as I am no RAID expert. If so, please correct me! - Maximum FW-Speed = 400Mbit/s, that's with the protocol overhead ~ 35Mb/s - Theoretical PCI-Bandwidth: 133 MB/s O.k., let's calculate a little bit, but only for large file disk writes, reads are probably not that easy to calculate: 1) RAID5: When writing a block, the actual data written is data*(5/4), but the data is spreaded over all 5 disks, therefore it should theoretically perform like a 4-disk RAID0. Practically there is probably a performance degration. 2) RAID10: When writing a block, the actual data written is data*2 as every data chunk is mirrored. The performance gain is like a 3-disk RAID0. So, theoretically the RAID5 should be faster but has a worse data reliability which could be improved by a hot spare. Anyway, due to the limitation of the FW, you will never gain a higher throughput than the ~ 35Mb/s, moreover keep in mind that the transfer speed between the HD-interface (cache) and the CPU can also never exceed this limit which degrades your performance, probably especially the read performance. When it comes to the PCI-bus, the load is higher with the RAID-10 solution, as the data that should be written is doubled. But it does not seem that the PCI-bus is a bottleneck in this system. Another thought to Gigabit Ethernet: 32-bit PCI Ethernet NIC's are known to be quite slow, often they provide not much more than 20-30Mb/s. Morover if you (mis)use your 32-bit PCI-bus for Gigabit Ethernet you will probably degrade your RAID performance as the PCI bus gets saturated. For Gigabit Ethernet you better use this Intel CSA-solution like found in the 875 chipsets or you have a motherboard with a 64-bit PCI or PCI-X bus (which is expensive). *Maybe* Nvidia also has an CSA-equivalent solution in its nForce3 Chipset, but I could not find any specs about this. Moreover I would also check the CPU-load which can also degrade performance as RAID5 needs CPU-speed and Gigabit-Ethernet (protocol etc.) can also use a lot of CPU. Best Regards, Hermann -- x1@aon.at GPG key ID: 299893C7 (on keyservers) FP: 0124 2584 8809 EF2A DBF9 4902 64B4 D16B 2998 93C7 - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
