On a Linux guest running under KVM, the root disk appears as /dev/vda rather than /dev/sda, and lsmod shows virtio_blk loaded. What does this indicate?
LPIC-1 Exam 101-500, objective 102. Linux installation and package management medium
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The options
Correct The guest is using a paravirtualized virtio block driver, which exchanges requests with the hypervisor directly instead of driving an emulated disk controller
Correct. virtio devices are the guest-side drivers of a virtualization-aware interface; the vd* naming is what the virtio block driver registers, as opposed to sd* for SCSI, SATA and USB storage.
Not correct The root filesystem is on an LVM logical volume
Wrong. Logical volumes appear under /dev/mapper/ with a vgname-lvname node and a /dev/vgname/lvname symlink, not as /dev/vda.
Not correct The system is a container rather than a virtual machine
Wrong. A container shares the host kernel and has no block device of its own to boot from; its root filesystem is a directory tree on the host, typically an overlay mount.
Not correct The disk is attached over the network by iSCSI
Wrong. An iSCSI LUN is presented by the kernel's SCSI stack and so appears as an ordinary sd* device.
Why
Full emulation makes the guest believe it has real hardware, so an unmodified driver works but every register access has to be trapped and emulated. Paravirtualized drivers instead accept that the guest knows it is virtualized and talk to the hypervisor through shared ring buffers, which is far cheaper; on KVM that family is virtio, with virtio_blk for disks, virtio_net for network interfaces and virtio_balloon for memory reclaim. The device naming is the visible clue: vda, vdb and so on for virtio disks.
Where this comes from
- Cited
- LPI exam objective 102.6
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