[9246] in linux-scsi channel archive
Re: concurrent commands to same LUN ?
daemon@ATHENA.MIT.EDU (Martin Peschke)
Wed Jul 26 05:10:29 2000
Date: Wed, 26 Jul 2000 11:01:15 +0200 (MET DST)
From: Martin Peschke <peschke@fh-brandenburg.de>
To: linux-scsi@vger.rutgers.edu
In-Reply-To: <87256926.005FB5BA.00@d53mta04h.boulder.ibm.com>
Message-ID: <Pine.GSO.4.10.10007261055390.29305-100000@zeus.fh-brandenburg.de>
MIME-Version: 1.0
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Yes, that is the scenario I think of. Queued commands seems to be the
right thing here. But which layer is responsible for determining
whether a particular SCSI command should be queued to the SCSI device and
which tag should be used? How does tagged queuing work in Linux?
Regards
Martin
On Mon, 24 Jul 2000 sjbuller@us.ibm.com wrote:
>
>
>
> I can not talk directly about select_queue_depths() and cmd_per_lun
> specifics, but yes, the protocol provides for command queueing in the
> LUN. Some large RAID devices utilize TCQ's of 80 or so. This allows
> the device to re-order the cmds for best performance (within limits).
>
> Multi-initiator in Linux is not that robust. It can work in SCSI, but it
> is also exposed to data corruption
> as the various hosts "fight" over who owns the bus and its devices. FC
> re-uses the SCSI
> middle layer driver.
>
> I've seen the function you describe refered to as data path optimization.
> A layer of code sits
> above the driver and routes commands to which ever adapter is least busy.
> If one path
> goes down, all traffic is then routed to the other path, providing
> redundancy as well.
>
> The other consideration in your scenario is the device itself. It has to
> support cmd queueing and
> would have to have multiple active ports.
>
> Regards,
> Steve Buller
> (std disclaimer: the comments above are my own and not those of my
> employer.)
>
>
> Martin Peschke <peschke@fh-brandenburg.de>@vger.rutgers.edu on 07/24/2000
> 09:07:39 AM
>
> Sent by: owner-linux-scsi@vger.rutgers.edu
>
>
> To: linux-scsi@vger.rutgers.edu
> cc:
> Subject: concurrent commands to same LUN ?
>
>
>
> I am just wondering about select_queue_depths() and cmd_per_lun.
> Is a HBA driver allowed to have more than one outstanding command at the
> same LUN? Or is it responsible for command serialization?
> I am just thinking about multiple paths in Fibre Channel environments
> because of several FC adapters within a single host connected to the same
> switched fabric. Are there cases where I can spread several commands to
> the same LUN over multiple ports at the same time to (maybe) increase
> performance?
> Thanks.
>
> Martin Peschke
>
>
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