> 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 Sorry for the long response time -- email got lost. top - 16:45:21 up 10 days, 17:41, 2 users, load average: 0.58, 0.17, 0.05 Tasks: 113 total, 2 running, 111 sleeping, 0 stopped, 0 zombie Cpu(s): 1.7% us, 87.7% sy, 6.3% ni, 0.0% id, 0.0% wa, 0.0% hi, 4.3% si Mem: 2061564k total, 2044784k used, 16780k free, 1193384k buffers Swap: 4257016k total, 552k used, 4256464k free, 24348k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 945 root 10 -5 0 0 0 S 44.2 0.0 7:27.73 md11_raid5 > > >> 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 > - 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
