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HW Raid poor io performance

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http://www.unix.com – Hello all We just built a storage cluster for our new xenserver farm. Using 3ware 9650SE raid controllers with 8 x 1TB WD sata disks in a raid 5, 256KB stripe size. While making first performance test on the local storage server using dd (which simulates the read/write access to the disk mostly equal as the iscsi target does it later) we see very strange performance values. Using the default dd (with the hardware reported block size of 512bytes) directly on the device (/dev/sdb) gives around 44MB/s write performance. Using dd with a 1MB blocksize (bs=1M) gives around 587MB/s write perfor (HowTos)