Bhyve storage improvements (was: Several bhyve quirks)

[Alexander Motin]

On 06.04.2015 23:38, Alexander Motin wrote:
>> I had some time to try it out today, but I'm still having issues:
> 
> I've just made experiment alike to your with making bhyve to work on top
> of GEOM device instead of preferable "dev" mode of ZVOL. And I indeed
> reproduced the problem. But the problem that I see is not related to the
> block size. The block size is reported to the guest correctly as 4K, and
> as I can see it works as such at least in FreeBSD guest.
> 
> The problem is in the way how bhyve inter-operates with block/GEOM
> devices. bhyve sends requests to the kernel with preadv()/pwritev()
> calls, specifying scatter/gather lists of buffer addresses provided by
> the guest. But GEOM code can not handle scatter/gather lists, only
> sequential buffer, and so single request is split into several. The
> problem is that splitting happens according to scatter/gather elements,
> and those elements in general case may not be multiple to the block
> size, that is fatal for GEOM and any block device.
> 
> I am not yet sure how to fix this problem. The most straightforward way
> is to copy the data at some point to collect elements of scatter/gather
> list into something sequential to pass to GEOM, but that requires
> additional memory allocation, and the copying is not free.  May be some
> cases could be optimized to work without copying but with some clever
> page mapping, but that seems absolutely not trivial.

I've committed the workaround to FreeBSD head at r281700.

-- 
Alexander Motin