I have Dell PowerEdge R 710 Server. Total number of HDD - 6. Each HDD is 3 TB in size.
1. Create disk group with 6 HDD - RAID level 6. Effective size will be 12 TB
2. Create one virtual disk - 250 GB
3. Created the second virtual disk in the same disk group with the remaining space
Will this be possible?? After this, I am planning to install Windows.
Yes, it is possible. Boot to CTRL-R, highlight the controller, hit Enter, select the disks, RAID level, and specify the SIZE ... be sure to check the Advanced and Initialize boxes, then OK. Install Windows, then create the second VD with the remaining space. You can do this using the OpenManage Server Administrator software (OMSA), or in CTRL-R.
You can install Windows on the 12GB disk (VD), but you will have to have UEFI enabled and installing a 64-bit OS, 2008 or later, and convert the "disk" to GPT prior to installation.
Out of curiosity. We set our systems up similar to what it sounds like you are doing. Are you going to install the OS on the 250GB Virtual Disk and use the rest of your space for storage?