[dm-devel] [LSF/MM ATTEND][LSF/MM TOPIC] Multipath redesign

[Hannes Reinecke]

On 01/14/2016 08:09 PM, Bart Van Assche wrote:
> On 01/13/2016 11:25 PM, Hannes Reinecke wrote:
>> On 01/13/2016 06:52 PM, Benjamin Marzinski wrote:
>>> On Wed, Jan 13, 2016 at 10:10:43AM +0100, Hannes Reinecke wrote:
>>>> c) implement block or scsi events whenever a remote port becomes
>>>>     unavailable. This removes the need of the 'path_checker'
>>>>     functionality in multipath-tools.
>>>
>>> I'm not convinced that we will be able to find out when paths
>>> come back
>>> online in all cases without some sort of actual polling. Again,
>>> I'd love
>>> this to be simpler, but asking all the types of storage we plan to
>>> support to notify us when they are up and down may not be realistic.
>>
>> Currently we have three main transports: FC, iSCSI, and SAS.
>
> Hello Hannes,
>
> Since several years the Linux SRP initiator driver also has reliable
> and efficient H.A. support. The IB spec supports port state change
> notifications. But whether or not port state information affects the
> path state should be configurable. Several IB users wouldn't like it
> if port state information would affect the path state because the
> time during which a port is down can be shorter than the time during
> which an IB HCA keeps retrying to send a packet.
>
Oooh, but of course I've forgotten SRP. Sorry, Bart; it's just not 
on my radar (what with me having no Infiniband equipment to speak of 
...)

But the above really sounds similar to the dev_loss_tmo mechanism we 
have on FC. Maybe it's worth looking into if we could have a similar 
mechanism on SRP.

The point here is that (on FC) we have the following flow of events:

Path loss
-> start dev_loss_tmo
-> rport set to 'blocked'
-> RSCN received
-> move to final rport state (online or gone)
-> unblock rport
-> stop dev_loss_tmo (if rport is online) or
-> dev_loss_tmo fires and removes rport

atm we're being notified once the port is moved to the final state, 
as that's when I/O continues or is being aborted and we're getting 
the I/O completion back.
With path events we could react to the actual path loss, and 
redirect I/O to another path directly when the path loss occurs.
But this really is a matter of policy; it might be that the path 
switch is taking long then the path interruption.
So this needs to be evaluated properly.
But at least we'll be notified allowing us to _do_ these kind of test.
ATM we don't really have a chance to do that.

I'm very willing to look at SRP to see if we can improve things there.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare at suse.de			               +49 911 74053 688
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