Michael Evans <mjevans1983@xxxxxxxxx> writes: > On Mon, Dec 14, 2009 at 1:06 AM, Matthieu Patou <mat@xxxxxxxxx> wrote: >> Hello Goswin, >>> Software raid1 will write twice as much data. That means twice as much >>> data goes over the system bus and into the controler cache. >>> Effectively you have halve the cache size. Maybe that is all you see. >>> >>> MfG >>> Goswin >>> >> Your idea seems logical, and I took a few hours today to verify it and it's >> the case as accessing the disk without software raid leads to almost the >> same result as with hardware raid. >> >> Matthieu. >> -- >> 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 >> > > Thinking about it, the doubling of data going over the system bus part > is correct. You must still push the operation out to each device's > buffer. However for -most- modern systems the system-bus will not be > the bottleneck for a reasonable number of drives. > > Further the later half which I'd skimmed over the first time is > utterly incorrect. Each drive would still only see the commands > targeting that drive. That should be virtually the same if not > identical to the single-drive case. The controler itself has cache. And that is shared between all drives. > The two most likely bottlenecks are single-drive write speed, and any > IO barriers that are used to ensure file system consistency in the > event of sudden interruption. MfG Goswin -- 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
