SoftRAID RAID-4 / RAID-5 Performance with Heavy CPU load

RAID-4 and RAID-5 offer fault tolerance against a drive failure. This fault tolerance derives from the use of parity data, which must be computed when disk writes are done. This parity calculation is performed by the CPU and so if the CPU is heavily loaded, things slow down for writes (parity is not computed for reads by SoftRAID).

SoftRAID will use one CPU core for every drive that is part of the RAID-4/RAID-5, hence a dual-core machine (or even 4 core) might not run as fast even if idle, and under load will definitely see degradation (for writes).

The important point is that most workloads neither utilize all CPUs nor does the workload involve a lot of writing. But some workloads might, so a RAID-0 stripe may be more appropriate (e.g., processing a video stream to a new format). Still, few workloads will write a gigabyte a second, so specifics are what matter.

Performance with RAID-4, idle machine vs heavy CPU load

8-core 3.3 GHz 2013 Mac Pro, 64GB memory,
7-drive RAID-4
Machine mostly idle
Iteration 	Write MiB/sec	Read MiB/sec
1    	     1014    	    1071    
2    	     1054    	    1077    
3    	     1020    	    1079    
4    	     1002    	    1104    
5    	     1031    	    1103    
6    	     1013    	    1068    
7    	      952    	    1098    
8    	      967    	    1122    
9    	      971    	    1114    
10    	      953    	    1114 
Slowest  	     952     	    1068    
Fastest  	     1054    	    1122    
Average  	     998     	    1095    
Median  	     1008    	    1101    
Range   	     102     	    54.4    
StdDev  	     35.0    	    19.9    
Machine heavily loaded: mt stress -t 8 -m 512M
Iteration 	Write MiB/sec	Read MiB/sec
1    	     487     	    1140    
2    	     486     	    1100    
3    	     480     	    1077    
4    	     468     	    1118    
5    	     447     	    1110    
6    	     439     	    1031    
7    	     431     	    1098    
8    	     438     	    1057    
9    	     424     	    1109    
10    	     424     	    1102    
Slowest  	     424     	    1031    
Fastest  	     487     	    1140    
Average  	     453     	    1094    
Median  	     443     	    1101    <=== 56% REDUCTION in WRITE SPEED
Range   	     63.3    	     109    
StdDev  	     25.6    	    31.5    

Performance with RAID-0, idle machine vs heavy CPU load

RAID-0 striping has very low CPU overhead (no parity to compute) and thus is all but unaffected by a heavy CPU load.

8-core 3.3 GHz 2013 Mac Pro, 64GB memory,
8 drive RAID-0
Machine mostly idle Iteration 	Write MiB/sec	Read MiB/sec
      1    	     1332    	    1357    
      2    	     1316    	    1351    
      3    	     1301    	    1315    
      4    	     1321    	    1314    
      5    	     1340    	    1320    
      6    	     1324    	    1346    
      7    	     1305    	    1348    
      8    	     1274    	    1329    
      9    	     1277    	    1358    
     10    	     1314    	    1293    
		
  Slowest  	     1274    	    1293    
  Fastest  	     1340    	    1358    
  Average  	     1310    	    1333    
   Median  	     1315    	    1338    
   Range   	     66.6    	    65.3    
   StdDev  	     21.8    	    22.3    
Machine heavily loaded: mt stress -t 8 -m 512M
Iteration 	Write MiB/sec	Read MiB/sec
      1    	     1312    	    1353    
      2    	     1261    	    1306    
      3    	     1257    	    1341    
      4    	     1322    	    1332    
      5    	     1289    	    1286    
      6    	     1327    	    1346    
      7    	     1328    	    1325    
      8    	     1276    	    1348    
      9    	     1291    	    1290    
     10    	     1285    	    1348    
		
  Slowest  	     1257    	    1286    
  Fastest  	     1328    	    1353    
  Average  	     1295    	    1328    
   Median  	     1290    	    1337    
   Range   	     71.2    	    67.3    
   StdDev  	     26.3    	    25.1