Re: [SPDK] BDEV & VBDEV

[Daniel Verkamp <daniel.verkamp at intel.com>]

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On 10/21/2016 06:31 AM, Kumaraparameshwaran Rathnavel wrote:
> Hi Guys,
>
> What is the difference between BDEV and VBDEV ?
>
> Thanking You,
> Param

Hi Param,

Regular bdevs expose raw block devices (NVMe namespaces, kernel block 
devices via libaio, etc.). They typically interface with a hardware 
device that needs to be polled periodically to check for completions.

A vbdev is a "virtual blockdev" that consumes one or more bdevs and 
produces another set of bdevs.  A vbdev can process each I/O and decide 
what to do with it - for example, it could send each I/O to an 
underlying bdev, or split a single request into multiple I/Os to several 
different bdevs to implement features like RAID, or even encrypt writes 
and decrypt reads to add a disk encryption layer.

We don't have any vbdev examples in the tree currently, but we plan to 
add an example vbdev that will read partition tables from a disk and 
expose each partition as another bdev that can be exposed as an iSCSI 
LUN or NVMe over Fabrics namespace.

Thanks,
-- Daniel