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

Last updated 2015-11-18 - Send Feedback
Related: Storage, Video, RAID, Thunderbolt

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    
Cycling

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
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