Raid 10 read speed does not scale with disk addition

We have 2 Dell boxes with the Perc H700 Raid Controller. One box has 6 x 600G disks configured as a single Raid 10 volume The other box has 12 x 600G disks configured as a single Raid 10 volume

I did a simple dd test by writing a file 2 x size of RAM ..I also avoided linux OS caching effects by writing another file the size of RAM before reading the original file I wrote.

I got similar write speeds in both boxes (about 350 -400MB/s)

I got a read speed of 590MB/s on the 6 disk box and 723 MB/sec on the 12 disk box.

I was curious if the doubling of disks should result in an approximate doubling of read speeds.

From serverfault RaidFan

• I guess you are reaching the speed limit of either your controllers hardware and/or the PCI Express connection.

  RaidFan : Thanks..that makes sense..so..the only way to get linear scaling is to have additional controllers. In this case..the 723MB/s seems to max out given the 6Gb/s interface speed.

  RaidFan : Would adding a second controller help double throughput ?

  TomTom : Only with software raid - hardly any raid controller I know of supports split controller raid arrays. Getting a proper controller would help - 12 drives should be on a SAS setup.
  From SvenW

• Try using different block sizes with dd. You're probably hitting an IOPS limit in your controller or driver.

  Also try using hdparm -t for read tests.

  From diq