Re: [RFC] [Last Rites] fc transport: extensions for fast fail and dev loss

[Mike Christie <michaelc@cs.wisc.edu>]

James Smart wrote:
> 
> 
> Christoph Hellwig wrote:
>>>   at (b): Minimally, we should terminate all active i/o requests marked
>>>           as type REQ_FASTFAIL. From an api perspective, driver support
>>>           for this is optional. And we must also assume that there will
>>>           be implementations which have to abort all i/o in order to
>>>           terminate those marked REQ_FASTFAIL. Is this acceptable ?
>>>           (it meets the "always" condition above)
>>>
>>>           Q: so far we've limited the io to those w/ REQ_FASTFAIL.
>>>             Would we ever want to allow a user to fast fail all i/o
>>>             regardless of the request flags ? (in case they flags
>>>             weren't getting set on all the i/o the user wanted to
>>>             see fail ?)
>>
>> I think we should fail all.  It's not like an unprivilegued process could
>> request FASTFAIL.  The administrator should know what she/he is doing.
> 
> Good.  All it is.
> 
>>>> - fast_loss_time recommendation:
>>>>  In discussing how a admin should set dev_loss_tmo in a multipathing
>>>>  environment, it became apparent that we expected the admin to know
>>>>  a lot. They had to know the transport type, what the minimum setting
>>>>  can be that still survives normal link bouncing, and they may even
>>>>  have to know about device specifics.  For iSCSI, the proper loss time
>>>>  may vary widely from session to session.
>>>>
>>>>  This attribute is an exported "recommendation" by the LLDD and
>>>> transport
>>>>  on what the lowest setting for dev_loss_tmo should be for a
>>>> multipathing
>>>>  environment. Thus, the admin only needs to cat this attribute to
>>>> obtain
>>>>  the value to echo into dev_loss_tmo.
>>> The only objection was from Christoph - wanting a utility to get/set
>>> this
>>> stuff. However, the counter was this attribute was still meaningful,
>>> as it
>>> was the conduit to obtain a recommendation from the transport/LLD.
>>>
>>> So - I assume this proceeds as is - with a change in it's description.
>>
>> I must say I'm still not happy with this.  It's really policy that we
>> try to keep out of the kernel.
> 
> Ok. I'll drop this. I don't think it was that important for FC. Mike
> Christie
> had some better arguments for iSCSI.
> 

For softwware iscsi we probably do not need kernel support since we do
so much setup in userspace we can easily have the admin pass in a
different argument for different defaults and we also have a way to set
different values and store them in userspace. So no big deal. For Hw
iscsi, we can work similar to FC so we should have no problem there too.