Alan M. Evans writes: : On Tue, 2007-09-18 at 18:26 +0200, Test wrote: : > This off course is very logical... : > : > Raid5 writes to all 3 disks at about the same time plus it has to write : > the crc/verification data which also causes some overhead. : > : > so the average speed = 55+71+75 / 3 = 67... : > : > So your speed measurement is correct... : : I wouldn't have expected such poor performance for *reads*, which is : what the OP complained about specifically. Even the web link you : provided below states: : : "The read performance of RAID 5 is almost as good as RAID 0 for : the same number of disks. Except for the parity blocks, the : distribution of data over the drives follows the same pattern as : RAID 0." : : So RAID5 should, presumably, be able to split the reads over multiple : disks and achieve much better than disk-average performance when : reading. That was roughly my thinking (being a RAID N00BIE) But even for writes, my thinking is (was?) as follows: If I write 100 MB of data to the RAID 5, then the 100 MB gets split (roughly) into a 50 MB piece for the 55MB/s disk, a 50 MB piece for the 71MB/s disk, and a 50 MB piece for the 75MB/s disk. Two of these pieces are (striped) data, one is parity. The slowest drive determines the time it takes to complete the write: T = max( 50MB/(55MB/s) , 50MB/(71MB/s) , 50MB/(75MB/s) ). So the entire 100 MB of data is written (neglecging parity calcs) in T = 0.909 seconds and the avg. data rate is 100MB/.909s = 110 MB/s. Where's my mistake? What are others who run software RAID 5 seeing compared to the individual partition speeds? Dean
