hum nice raid10 and raid0 are single thread too? 2012/5/21 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>: > On 5/21/2012 1:54 PM, Roberto Spadim wrote: >> hum, does anyone could explain what a 'multi thread' version of raid1 >> could be implemented? >> for example, how to scale it? and why this new implementation could >> scale it better > > I just did below. You layer a stripe over many RAID 1 pairs. A single > md RAID 1 pair isn't enough to saturate a single core so there is no > gain to be had by trying to thread the RAID 1 code. > > -- > Stan > > >> 2012/5/21 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>: >>> On 5/21/2012 10:20 AM, CoolCold wrote: >>>> On Sat, May 12, 2012 at 2:28 AM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: >>>>> On 5/11/2012 3:16 AM, Daniel Pocock wrote: >>>>> >>>> [snip] >>>>> That's the one scenario where I abhor using md raid, as I mentioned. At >>>>> least, a boot raid 1 pair. Using layered md raid 1 + 0, or 1 + linear >>>>> is a great solution for many workloads. Ask me why I say raid 1 + 0 >>>>> instead of raid 10. >>>> So, I'm asking - why? >>> >>> Neil pointed out quite some time ago that the md RAID 1/5/6/10 code runs >>> as a single kernel thread. Thus when running heavy IO workloads across >>> many rust disks or a few SSDs, the md thread becomes CPU bound, as it >>> can only execute on a single core, just as with any other single thread. >>> >>> This issue is becoming more relevant as folks move to the latest >>> generation of server CPUs that trade clock speed for higher core count. >>> Imagine the surprise of the op who buys a dual socket box with 2x 16 >>> core AMD Interlagos 2.0GHz CPUs, 256GB RAM, and 32 SSDs in md RAID 10, >>> only to find he can only get a tiny fraction of the SSD throughput. >>> Upon investigation he finds a single md thread peaking one core while >>> the rest are relatively idle but for the application itself. >>> >>> As I understand Neil's explanation, the md RAID 0 and linear code don't >>> run as separate kernel threads, but merely pass offsets to the block >>> layer, which is fully threaded. Thus, by layering md RAID 0 over md >>> RAID 1 pairs, the striping load is spread over all cores. Same with >>> linear, avoiding the single thread bottleneck. >>> >>> This layering can be done with any md RAID level, creating RAID50s and >>> RAID60s, or concatenations of RAID5/6, as well as of RAID 10. >>> >>> And it shouldn't take anywhere near 32 modern SSDs to saturate a single >>> 2GHz core with md RAID 10. It's likely less than 8 SSDs, which yield >>> ~400K IOPS, but I haven't done verufication testing myself at this point. >>> >>> -- >>> 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 >> >> >> > > -- > 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 -- Roberto Spadim Spadim Technology / SPAEmpresarial -- 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
