/dev/sda: Samsung SSD 850 EVO 2TB
/dev/sdb: Samsung SSD 850 EVO 2TB
/dev/sdc: ST2000DX002-2DV164
/dev/sdd: ST2000DX002-2DV164

normally i would expect that for heavy read-IO the two HDD drives would sleep and the two SSD working more or less like a RAID0 but as example running a btrfs-srub within a virtual machine on a 1.5 TB vdisk you can hear the rotating disks clearly and there are also unexpected lags while a disk IO auf around 700 MB/sec shows that it's basicly working

pretty sure the lags are coming because read-access is also spread to HDD - why is that?

/dev/md2:
        Version : 1.1
  Creation Time : Wed Jun  8 13:10:56 2011
     Raid Level : raid10
     Array Size : 3875222528 (3695.70 GiB 3968.23 GB)
  Used Dev Size : 1937611264 (1847.85 GiB 1984.11 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Sat Feb 11 15:10:31 2017
          State : active
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

Name : localhost.localdomain:2 (local to host localhost.localdomain)
           UUID : ea253255:cb915401:f32794ad:ce0fe396
         Events : 1818029

    Number   Major   Minor   RaidDevice State
4 8 35 0 active sync set-A writemostly /dev/sdc3
       5       8       19        1      active sync set-B   /dev/sdb3
7 8 51 2 active sync set-A writemostly /dev/sdd3
       6       8        3        3      active sync set-B   /dev/sda3
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