Subject: Re: [RFC][PATCH 0/9] Network receive deadlock prevention for NBD

From: Daniel Phillips <[email-masked]>
Date: Wed, 16 Aug 2006 21:48:37 -0700

Evgeniy Polyakov wrote:
> On Sun, Aug 13, 2006 at 01:16:15PM -0700, Daniel Phillips ([email-masked]) wrote:
>>Indeed.  The rest of the corner cases like netfilter, layered protocol and
>>so on need to be handled, however they do not need to be handled right now
>>in order to make remote storage on a lan work properly.  The sane thing for
>>the immediate future is to flag each socket as safe for remote block IO or
>>not, then gradually widen the scope of what is safe.  We need to set up an
>>opt in strategy for network block IO that views such network subsystems as
>>ipfilter as not safe by default, until somebody puts in the work to make
>>them safe.
> 
> Just for clarification - it will be completely impossible to login using 
> openssh or some other priveledge separation protocol to the machine due
> to the nature of unix sockets. So you will be unable to manage your
> storage system just because it is in OOM - it is not what is expected
> from reliable system.

The system is not OOM, it is in reclaim, a transient condition that will be
resolved in normal course by IO progress.  However you raise an excellent
point: if there is any remote management that we absolutely require to be
available while remote IO is interrupted - manual failover for example -
then we must supply a means of carrying out such remote administration, that
is guaranteed not to deadlock on a normal mode memory request.  This ends up
as a new network stack feature I think, and probably a theoretical one for
the time being since we don't actually know of any such mandatory login
that must be carried out while remote disk IO is suspended.

>>But really, if you expect to run reliable block IO to Zanzibar over an ssh
>>tunnel through a firewall, then you might also consider taking up bungie
>>jumping with the cord tied to your neck.
> 
> Just pure openssh for control connection (admin should be able to
> login).

And the admin will be able to, but in the cluster stack itself we don't
bless such stupidity as emailing an admin to ask for a login in order to
break a tie over which node should take charge of DLM recovery.

Regards,

Da niel