Снижение производительности записи при зеркалировании двух массивов RAID 0

На сервере CentOS 7 с 4 твердотельными накопителями я создал два массива RAID 0 с mdadm. Оба были отформатированы с помощью ext4 и смонтированы в разных каталогах.

Я проверил их, используя fio и получил следующие результаты для случайных записей:

randomwrites: (g=0): rw=randwrite, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=64
fio-2.2.8
Starting 1 process
Jobs: 1 (f=1)
randomwrites: (groupid=0, jobs=1): err= 0: pid=21814: Sat May 21 15:47:19 2016
  write: io=1024.0MB, bw=696266KB/s, iops=174066, runt=  1506msec
  cpu          : usr=9.04%, sys=89.37%, ctx=3803, majf=0, minf=27
  IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, >=64=100.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
     issued    : total=r=0/w=262144/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
     latency   : target=0, window=0, percentile=100.00%, depth=64

Run status group 0 (all jobs):
  WRITE: io=1024.0MB, aggrb=696265KB/s, minb=696265KB/s, maxb=696265KB/s, mint=1506msec, maxt=1506msec

Disk stats (read/write):
    dm-0: ios=0/243409, merge=0/0, ticks=0/81182, in_queue=81298, util=93.32%, aggrios=0/262144, aggrmerge=0/0, aggrticks=0/0, aggrin_queue=0, aggrutil=0.00%
    md67: ios=0/262144, merge=0/0, ticks=0/0, in_queue=0, util=0.00%, aggrios=0/131068, aggrmerge=0/4, aggrticks=0/43552, aggrin_queue=43553, aggrutil=92.14%
  sda: ios=0/130872, merge=0/2, ticks=0/43363, in_queue=43381, util=92.02%
  sdb: ios=0/131264, merge=0/6, ticks=0/43741, in_queue=43725, util=92.14%

Затем я создал массив RAID 1 с обоими этими массивами RAID 0 и снова провел тесты.

randomwrites: (g=0): rw=randwrite, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=64
fio-2.2.8
Starting 1 process
Jobs: 1 (f=1): [w(1)] [-.-% done] [0KB/473.1MB/0KB /s] [0/121K/0 iops] [eta 00m:00s]
randomwrites: (groupid=0, jobs=1): err= 0: pid=22598: Sat May 21 16:00:55 2016
  write: io=1024.0MB, bw=482770KB/s, iops=120692, runt=  2172msec
  cpu          : usr=8.66%, sys=61.40%, ctx=73489, majf=0, minf=28
  IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, >=64=100.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
     issued    : total=r=0/w=262144/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
     latency   : target=0, window=0, percentile=100.00%, depth=64

Run status group 0 (all jobs):
  WRITE: io=1024.0MB, aggrb=482769KB/s, minb=482769KB/s, maxb=482769KB/s, mint=2172msec, maxt=2172msec

Disk stats (read/write):
    dm-0: ios=0/259433, merge=0/0, ticks=0/134614, in_queue=135499, util=95.95%, aggrios=0/262144, aggrmerge=0/0, aggrticks=0/0, aggrin_queue=0, aggrutil=0.00%
    md69: ios=0/262144, merge=0/0, ticks=0/0, in_queue=0, util=0.00%, aggrios=0/262145, aggrmerge=0/0, aggrticks=0/0, aggrin_queue=0, aggrutil=0.00%
    md67: ios=0/262145, merge=0/0, ticks=0/0, in_queue=0, util=0.00%, aggrios=0/131075, aggrmerge=0/0, aggrticks=0/66948, aggrin_queue=66958, aggrutil=94.64%
  sda: ios=0/130878, merge=0/0, ticks=0/66753, in_queue=66744, util=94.64%
  sdb: ios=0/131273, merge=0/0, ticks=0/67143, in_queue=67172, util=94.59%
    md68: ios=0/262145, merge=0/0, ticks=0/0, in_queue=0, util=0.00%, aggrios=0/131075, aggrmerge=0/0, aggrticks=0/68108, aggrin_queue=68114, aggrutil=94.68%
  sdc: ios=0/130878, merge=0/0, ticks=0/67942, in_queue=67928, util=94.68%
  sdd: ios=0/131273, merge=0/0, ticks=0/68274, in_queue=68300, util=94.68%

Как видите, массив RAID 0 выполнен на 174066 иопс в то время как RAID 1 по двум RAID 0 давал только 120692 iops. В чем причина падения скорости записи?

Планировщик ввода-вывода настроен на нет для всех 4 SSD.