Re: [Qemu-devel] Question on virtio disk maximum index and maximum partition

[Stefan Hajnoczi]

On Wed, Jun 1, 2011 at 2:06 AM, Wei Liu <address@hidden> wrote:
> I'm programming for virtio disk support in Xen tool stack. I would
> like to know the limitation of virtio disk.

I'm interested what you are implementing - a virtio-blk backend for
Xen (which would basically mean vhost-blk)?

> That is, what's the maximum number of disks supported through virtio
> bus, and what's the maximum number of partitions supported per disk.

virtio-blk as used by KVM is exposed as a virtio PCI adapter.  There
is a 1:1 mapping between virtio-blk, PCI adapters, and block devices
being presented by QEMU:

1 virtio-blk device in guest == 1 virtio-pci adapter in guest == 1
block device in QEMU

The maximum number is really limited by the PCI bus, not virtio.  In
terms of coding, you should try not to impose a hard limit at all.

Partitions are not at the virtio-blk level.  The guest operating
system will see the virtio-blk disk and scan its partition table to
determine which partitions are available.  The limit then depends on
the partitioning scheme that you use (legacy boot record, GPT, etc).

> If I'm posting to the wrong list, please point me the right direction.

You might also be interested in
<address@hidden> for general virtio
discussion.

Stefan