[LSF/MM ATTEND][LSF/MM TOPIC] Multipath redesign

[Hannes Reinecke]

On 01/13/2016 11:50 AM, Sagi Grimberg wrote:
>
>> Hi all,
>>
>> I'd like to attend LSF/MM and would like to present my ideas for a
>> multipath redesign.
>>
>> The overall idea is to break up the centralized multipath handling in
>> device-mapper (and multipath-tools) and delegate to the appropriate
>> sub-systems.
>
> I agree that would be very useful. Great topic. I'd like to attend
> this talk as well.
>
>>
>> Individually the plan is:
>> a) use the 'wwid' sysfs attribute to detect multipath devices;
>>     this removes the need of the current 'path_id' functionality
>>     in multipath-tools
>
> CC'ing Linux-nvme,
>
> I've recently looked at multipathing support for nvme (and nvme over
> fabrics) as well. For nvme the wwid equivalent is the nsid (namespace
> identifier). I'm wandering if we can have better abstraction for
> user-space so it won't need to change its behavior for scsi/nvme.
> The same applies for the the timeout attribute for example which
> assumes scsi device sysfs structure.
>
My idea for this is to lookup the sysfs attribute directly from 
multipath-tools. As such we would need to have some transport 
information in multipath so that we know where to find it.
And with that we should easily able to accomodate NVMe, provided the 
nsid is displayed somewhere in sysfs.

>> b) leverage topology information from scsi_dh_alua (which we will
>>     have once my ALUA handler update is in) to detect the multipath
>>     topology. This removes the need of a 'prio' infrastructure
>>     in multipath-tools
>
> This would require further attention for nvme.
>
Indeed. But then I'm not sure how multipath topology would be 
represented in NVMe; we would need some way of transmitting the 
topology information.
Easiest would be to leverage VPD device information; so we only need 
the equivalent of REPORT TARGET PORT GROUPS to implement an 
ALUA-like scenario.

>> 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'd prefer if we'd have it in the block layer so we can have it for all
> block drivers. Also, this assumes that port events are independent of
> I/O. This assumption is incorrect in SRP for example which detects port
> failures only by I/O errors (which makes path sensing a must).
>
That's what I though initially, too.
But then we're facing a layering issue:
The path events are generated at the _transport_ level.
So for SCSI we have to do a redirection
transport layer->scsi layer->scsi ULD->block device
requiring us to implement for sets of callback functions.
Which I found rather pointless (and time consuming), so I opted for 
scsi events (like we have for UNIT ATTENTION) instead.

However, even now we're having two sets of events (block events and 
scsi events) with a certain overlap, so this really could do with a 
cleanup.

>> d) leverage these events to handle path-up/path-down events
>>     in-kernel
>> e) move the I/O redirection logic out of device-mapper proper
>>     and use blk-mq to redirect I/O. This is still a bit of
>>     hand-waving, and definitely would need discussion to figure
>>     out if and how it can be achieved.
>>     This is basically the same topic Mike Snitzer proposed, but
>>     coming from a different angle.
>
> Another (adjacent) topic is multipath performance with blk-mq.
>
> As I said, I've been looking at nvme multipathing support and
> initial measurements show huge contention on the multipath lock
> which really defeats the entire point of blk-mq...
>
> I have yet to report this as my work is still in progress. I'm not sure
> if it's a topic on it's own but I'd love to talk about that as well...
>
Oh, most definitely. There are some areas in blk-mq which need to be 
covered / implemented before we can even think of that (dynamic 
queue reconfiguration and disabled queue handling being the most 
prominent).

_And_ we have the problem of queue mapping (one queue per ITL nexus?
one queue per hardware queue per ITL nexus?) which might quickly 
lead to a queue number explosion if we've not careful.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare at suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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