linux-kernel - Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

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Date:   Thu, 11 Oct 2018 10:59:11 +0100
From:   John Garry <john.garry@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
CC:     <jejb@...ux.vnet.ibm.com>, <linuxarm@...wei.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>,
        chenxiang <chenxiang66@...ilicon.com>
Subject: Re: [PATCH 0/7] hisi_sas: Misc bugfixes and an optimisation patch

On 11/10/2018 07:36, Christoph Hellwig wrote:
> On Wed, Oct 10, 2018 at 09:58:25PM -0400, Martin K. Petersen wrote:
>>> This is because the IPTT index must be a unique value per HBA. However,
>>> if we switched to SCSI MQ, the block layer tag becomes unique per queue,
>>> and not per HBA.
>>
>> That doesn't sound right.
>

Hmmm, well this is what I was told ...

> blk-mq tags are always per-host (which has actually caused problems for
> ATA, which is now using its own per-device tags).
>

So, for example, if Scsi_host.can_queue = 2048 and 
Scsi_host.nr_hw_queues = 16, then rq tags are still in range [0, 2048) 
for that HBA, i.e. invariant on queue count?

> .
>

Thanks,
John