Multiple SATA HDD in a Virtual Box divides the speed or io rate

[harishpathangay]

Hi,
I have a vbox with 2 CPUs configured [Laptop is Intel 3825U 2 Cores 4 Threads] with 3 SATA drives.
/dev/sda1 - Linux OS
/dev/sdb1 - xfs file system for storing data
/dev/sdc1 - xfs file system for storing data

My Database application is writing into sdb1 with IO rate 14-15 Mbps. I did not limit any bandwidths.
My Database application is writing into sdc1 with IO rate 14-15 Mbps. I did not limit any bandwidths.

In my Database application I enable parallel writing so that data is written into sdb1 and sdc1 parallel. I am expecting the task to complete in half time.

I am observing it is doing parallel writes [iostat command], but the io rate is half only, meaning I get only 7-8 Mbps together. So 3.5Mbps per disk in a parallel fashion. Why the IO write rate is getting proportionally divided by number of disks. How to configure such that I will have at least reasonable write rates in parallel fashion.

thanks,

harish pathangay