breakage in blkfront with ring_pages > 1

[Colin Percival]

On 07/06/11 15:55, Justin T. Gibbs wrote:
> On 7/5/11 10:14 PM, Colin Percival wrote:
>> On 07/05/11 19:42, Colin Percival wrote:
>>> Yep. Mess or not, shall I go ahead with having a loader tunable
> control this,
>>> or can you think of a better solution?
>>
>> Does anyone object to the attached patch? It keeps the differing
> behaviour to
>> a minimum -- we MUST set ring-ref with a FreeBSD blkback, and we MUST
> NOT set
>> it with a linux blkback -- but otherwise errs in the direction of
> setting more
>> variables than are needed, to maximize the possibility of a future blkback
>> being compatible with both blkback_is_linux=0 and blkback_is_linux=1.
> 
> It would be better to just change the FreeBSD blkback driver to be
> compatible with the RedHat convention.

Fine with me, but that will of course break compatibility between pre- and
post- patch versions of FreeBSD.  Aside from you, how many people use the
FreeBSD blkback driver?

> I'm still unclear on why the current FreeBSD blkfront driver believes
> that it can use more than one page in your configuration given that the
> RedHat blkfront doesn't advertise this capability in a way that the FreeBSD
> blkfront understands (max-ring-pages isn'te set by blkback).  Did you do
> something to force blkfront to use more than one page?

I'm seeing max-ring-pages set to 4.  I don't know what tree EC2 is using on
their Dom0 -- I've heard rumours that there's a lot of RedHat going on behind
the scenes, but given Amazon's size it's entirely possible that they got a
blkback which hasn't been released publicly.

I'm sent an email asking where their blkback came from; I'll report back if/when
I know something.

-- 
Colin Percival
Security Officer, FreeBSD | freebsd.org | The power to serve
Founder / author, Tarsnap | tarsnap.com | Online backups for the truly paranoid