在有4个SSD的CentOS 7服务器上，我用mdadm创build了两个RAID 0arrays。 两者都用ext4格式化并安装在单独的目录中。
我使用fio进行了基准testing，并得到了随机写入的以下结果：
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%
然后我用这两个RAID 0arrays创build了一个RAID 1arrays，然后再次运行testing。
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%
正如你可以看到RAID 0arrays在174066 iops上执行，而RAID 1在两个RAID 0上只能提供120692 iops 。 写入性能下降的原因是什么？
IO调度程序被设置为全部4个SSD的noop 。
一个软件RAID1需要复制每个数据块，在SB和SATA链路上有效地传输两次。 这意味着在某些时候，由于总线拥塞，在使用高性能存储驱动程序（在您的情况下，SSD）时，可以观察到显着的IOPS降低。
尝试增加I / O队列长度和/或切换到截止date调度程序，因为这两个更改都会增加I / O合并的可能性，从而减less总线拥塞。