> On Jan 24, 2020, at 3:43 AM, Vladislav Bolkhovitin <v...@vlnb.net> wrote:
>
>
>> ...
>
> From my old iSCSI target development days, MS is fundamentally not
> friendly to multi-queue, because it requires by the iSCSI spec to
> preserve order of commands inside the session across multiple
> connections. Commands serialization => shared lock or atomic => no
> multi-queue benefits.
>
> Hence, usage of MS for multi-queue would be beneficial only if to drop
> (aka violate) this iSCSI spec requirement.
>
> Just a small reminder. I have not looked in the updated iSCSI spec for a
> while, but don't remember this requirement was anyhow eased there.
>
> In any case, multiple iSCSI sessions per block level "session" would
> always be another alternative that would require virtually zero changes
> in open-iscsi and in-kernel iSCSI driver[1] as opposed to complex
> changes required to start supporting MS in it as well as in many iSCSI
> targets around that currently do not[2]. If I would be working on iSCSI
> MQ, I would consider this as the first and MUCH more preferable option.
>
> Vlad
>
> 1. Most likely, completely zero.
> 2. Where requirement to preserve commands order would similarly kill all
> the MQ performance benefits.
My reaction, from a similar background, matches yours. iSCSI makes things
quite hard by requiring ordering across the connections that make up a session.
That discourages implementation of multi-connection support in targets (it's
optional). In some cases, it entirely rules it out; for example, in the
EqualLogic storage arrays it would be pretty useless to support
multi-connection since the connections could not be spread over multiple
arrays, and for that reason we ruled out that feature.
By contrast, MPIO (several independent sessions used by the storage stack as a
wider and/or more fault tolerant pipe to the storage) requires essentially no
work at the target and gives at least as much benefit as MCS for a lot less
work.
paul
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