linux-kernel - Re: [PATCH] nvme-multipath: implement active-active round-robin path selector

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Date:   Wed, 4 Apr 2018 08:30:27 -0600
From:   Keith Busch <keith.busch@...el.com>
To:     "Eric H. Chang" <echang@...com>
Cc:     Christoph Hellwig <hch@....de>, Baegjae Sung <baegjae@...il.com>,
        "axboe@...com" <axboe@...com>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme-multipath: implement active-active round-robin
        path selector

On Fri, Mar 30, 2018 at 09:04:46AM +0000, Eric H. Chang wrote:
> We internally call PCIe-retimer as HBA. It's not a real Host Bus Adapter that translates the interface from PCIe to SATA or SAS. Sorry for the confusion.

Please don't call a PCIe retimer an "HBA"! :)

While your experiment is setup to benefit from round-robin, my only
concern is it has odd performance in a real world scenario with
IO threads executing in different nodes. Christoph's proposal will
naturally utilize both paths optimally there, where round-robin will
saturate node interlinks.

Not that I'm against having the choice; your setup probably does represent
real use. But if we're going to have multiple choice, user documentation
on nvme path selectors will be useful here.