disk io rate is splitting in half between multiple disks? any setting?

[harishpathangay]

hi,
I have netrunner/Debian on Vbox 6.0.4
Configured 2 Additional virtual HDD [sdb,sdc] of 512MB each; Linux is installed in separate virtual HDD sda of 12GB size.
Configured LVM with linear volumes using physical devices [sdb,sdc]. I am getting about 60,000KB/sec write rate.
remove the volume.
Configured LVM with striped volumes using physical devices [sdb,sdc]. I am getting about 30,000KB/sec write rate.
You can observe the write rate is coming down, so there is no improvement in my write activity in terms of elapsed time. it is going to be same.
remove the volume.
Configured LVM with striped volumes using physical devices [sdb,sdc,sdd]. I am getting about 20,000KB/sec write rate.
You can observe the write rate is coming down, so there is no improvement in my write activity in terms of elapsed time. it is going to be same again.

Should I enable something for getting io rates not getting split like that. Issue observed with both SATA and SCSI attachments of hard disk.

thanks,
harish pathangay