On Monday August 28, raid@xxxxxxxxxxx wrote: > This might be a dumb question, but what causes md to use a large amount of > cpu resources when reading a large amount of data from a raid1 array? I assume you meant raid5 there. md/raid5 shouldn't use that much CPU when reading. It does use more than raid0 as it reads data in the stripe-cache and then copies the data from the stripe cache into the read-buffer. But I wouldn't expect that to come anywhere near 50%. Are you really seeing 'raid5d' using 50% of CPU in 'top' or similar? NeilBrown > Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md > enhancements to later versions; I had some other unrelated issues and > rolled back to one I've run on for several months). > > A given 7-disk raid0 array can read 450MB/s (using cat > null) and use > virtually no CPU resources. (Although cat and kswapd use quite a bit [60%] > munching on the data) > > A raid5 array on the same drive set pulls in at 250MB/s, but md uses > roughly 50% of the CPU (the other 50% is spent dealing with the data, > saturating the processor). > > A consistency check on the raid5 array uses roughly 3% of the cpu. It is > otherwise ~97% idle. > md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0] > 248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU] > [==============>......] resync = 72.2% (29976960/41495808) > finish=3.7min speed=51460K/sec > (~350MB/s aggregate throughput, 50MB/s on each device) > > Just a friendly question as to why CPU utilization is significantly > different between a check and a real-world read on raid5? I feel like if > there was vm overhead getting the data into userland, the slowdown would > be present in raid0 as well. I assume parity calculations aren't done on a > read of the array, which leaves me at my question. > > Thanks, > Rob > > - > 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 -- VGER BF report: U 0.5 - 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
