On Mon, Apr 09, 2007 at 06:53:26AM -0400, Justin Piszcz wrote: > Using 2 threads made no difference either. > > It was not until I did 3 simultaneous copies that I saw 110-130MB/s > through vmstat 1, until then, it only used one drive, even with two cp's, > how come it needs to be three or more?
Because, as I understand it, it's an optimisation, not a rule. Quoting from the manpage (md): "Once initialised, each device in a RAID1 array contains exactly the same data. Changes are written to all devices in parallel. Data is read from any one device. The driver attempts to distribute read requests across all devices to maximise performance." The key word here is "attempts". I looked a while ago over the source code and IIRC it says that it tries to direct a read request to the drive whose head is closest to the requested sector, or if not possible a random drive. To me, this seems a good strategy, which optimises server-type workloads. regards, iustin - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
