> Rob Bray 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? >>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. >> >> > What are you stripe and cache sizes? > > -- > bill davidsen <davidsen@xxxxxxx> > CTO TMR Associates, Inc > Doing interesting things with small computers since 1979 > > - > 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 > 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] stripe_cache_size = 256 I've tried increasing it with the same result - 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
