Re: [LSF/MM TOPIC] iSCSI MQ adoption via MCS discussion

[Vladislav Bolkhovitin <vst@vlnb.net>]

Sagi Grimberg wrote on 01/08/2015 05:45 AM:
>> RFC 3720 namely requires that iSCSI numbering is
>> session-wide. This means maintaining a single counter for all MC/S
>> sessions. Such a counter would be a contention point. I'm afraid that
>> because of that counter performance on a multi-socket initiator system
>> with a scsi-mq implementation based on MC/S could be worse than with the
>> approach with multiple iSER targets. Hence my preference for an approach
>> based on multiple independent iSER connections instead of MC/S.
> 
> So this comment is spot on the pros/cons of the discussion (we might want to leave
> something for LSF ;)).
> MCS would not allow a completely lockless data-path due to command
> ordering. On the other hand implementing some kind of multiple sessions
> solution feels somewhat like a mis-fit (at least in my view).
> 
> One of my thoughts about how to overcome the contention on commands
> sequence numbering was to suggest some kind of negotiable "relaxed
> ordering" mode but of course I don't have anything figured out yet.

Linux SCSI/block stack neither uses, nor guarantees any commands order. Applications
requiring commands order enforce it by queue draining (i.e. wait until all previous
commands finished). Hence, MC/S enforced commands order is an overkill, which
additionally coming with some non-zero performance cost.

Don't do MC/S, do independent connections. You know the KISS principle. Memory overhead
to setup the extra iSCSI sessions should be negligible.

Vlad