Running "rados bench" on a SAS pool and SATA pool shows increased performance for the SATA pool, why?

Issue

• Running a "rados bench" on two pools based on SAS disks and SATA disks respectively, shows better performance for the pool on SATA disks.

• The setup consists of four OSD nodes with two SAS and SATA disks each.

• All the network is on 10GB.

• Two CRUSH rulesets, one for the SATA disks, and one for the SAS disks.

• Two pools, one based on the SAS ruleset, and the other based on the SATA ruleset.

• From a "rados bench", the pool on the SATA ruleset seems faster.

• On the pool based on the SAS disks:

# rados bench -p <pool_sas> 10 write
 Maintaining 16 concurrent writes of 4194304 bytes for up to 10 seconds or 0 objects
   sec Cur ops   started  finished  avg MB/s  cur MB/s  last lat   avg lat
     0       0         0         0         0         0         -         0
     1      15       146       131   523.875       524  0.110662  0.116181
     2      16       279       263   525.904       528  0.156652  0.117451
     3      16       418       402   535.914       556  0.116001  0.117155
     4      16       559       543   542.916       564  0.129779  0.116175
     5      16       693       677   541.521       536  0.123408  0.116826
     6      16       834       818   545.256       564  0.115398  0.116071
     7      16       981       965   551.354       588  0.128176  0.115259
     8      16      1114      1098   548.928       532 0.0536508  0.115637
     9      16      1255      1239   550.596       564  0.118188  0.115488
    10      16      1402      1386   554.329       588  0.124998  0.114782
 Total time run:         10.149064
Total writes made:      1403
Write size:             4194304
Bandwidth (MB/sec):     552.957

Stddev Bandwidth:       168.646
Max bandwidth (MB/sec): 588
Min bandwidth (MB/sec): 0
Average Latency:        0.115462
Stddev Latency:         0.0259277
Max latency:            0.329887
Min latency:            0.0396323

• On the pool based on the SATA disks:

# rados bench -p <pool_sata> 10 write
 Maintaining 16 concurrent writes of 4194304 bytes for up to 10 seconds or 0 objects
   sec Cur ops   started  finished  avg MB/s  cur MB/s  last lat   avg lat
     0       0         0         0         0         0         -         0
     1      16       244       228   911.257       912 0.0759171 0.0680092
     2      16       476       460   919.567       928 0.0787337 0.0684072
     3      16       700       684    911.56       896  0.069912 0.0695247
     4      16       923       907   906.646       892 0.0628496 0.0699802
     5      16      1143      1127   901.293       880 0.0827047 0.0703452
     6      16      1368      1352   901.057       900  0.100978 0.0706611
     7      16      1563      1547   883.755       780 0.0679113 0.0719268
     8      16      1765      1749   874.275       808   0.10753 0.0728132
     9      16      1994      1978   878.897       916 0.0504995 0.0725023
    10      16      2216      2200   879.799       888 0.0722877  0.072449
Total time run:         10.061829
Total writes made:      2217
Write size:             4194304
Bandwidth (MB/sec):     881.351

Stddev Bandwidth:       269.197
Max bandwidth (MB/sec): 928
Min bandwidth (MB/sec): 0
Average Latency:        0.0725998
Stddev Latency:         0.0199631
Max latency:            0.180094
Min latency:            0.0280216