why it's vda
Because it's a virtual disk. A cloud server
can be run on a hypervisor. The role of a hypervisor is to control the capacity of operating systems so it is allocated where needed. With cloud hosting there are multiple cloud servers which are available to each particular client. This allows computing resource to be dedicated to a particular client if and when it is necessary. http://www.interoute.com/what-are-cloud-servers
You system is a compartmentalized virtual guest OS run by a hypervisor inside a larger actual host OS system somewhere. In this case, the hypervisor is evidently QEMU:
Attached devices:
Host: scsi1 Channel: 00 Id: 00 Lun: 00
Vendor: QEMU Model: QEMU DVD-ROM
QEMU supports the use of virtio, whereby transparently virtual network and block devices can be created. Your "drive" is actually a disk image file on a real hard disk; when your system boots up this is image file is used to create the drive.
You don't have access to the actual hardware because that's not what you paid for. The purpose of virtual disks is to simplify some issues and increase performance for hypervisor systems.
It is possible for a hypervisor to simulate some kind of real hard drive so it would appear to be such inside the guest system. However, there's not much point to this, since it still isn't real (it would be a little absurd for the host to use actual drives, or even actual partitions, for each guest); doing this also requires more work for the host (= lower performance for the guest). Instead, the virtio drive is intended to work like a real disk for general I/O purposes and not waste resources simulating anything else. Any physical characteristics it reports are arbitrary -- it doesn't actually have any -- and don't bother trying to run smartctl etc. on it. Your current root filesystem mount options have probably been chosen by whoever runs the host; you may want to contact them before you contemplate changes.
