Date: Tue, 10 Sep 2002 12:26:50 -0700
From: Patrick Mansfield <patmans@us.ibm.com>
To: Lars Marowsky-Bree <lmb@suse.de>
Cc: linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org
Subject: Re: [RFC] Multi-path IO in 2.5/2.6 ?
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On Tue, Sep 10, 2002 at 03:16:06PM +0200, Lars Marowsky-Bree wrote:
> On 2002-09-09T17:08:47,
>    Patrick Mansfield <patmans@us.ibm.com> said:

> > Yes negotiation is at the adapter level, but that does not have to be tied
> > to a Scsi_Device. I need to search for Scsi_Device::hostdata usage to
> > figure out details, and to figure out if anything is broken in the current
> > scsi multi-path code - right now it requires the same adapter drivers be
> > used and that certain Scsi_Host parameters are equal if multiple paths
> > to a Scsi_Device are found.
> 
> This seems to be a serious limitation. There are good reasons for wanting to
> use different HBAs for the different paths.

What reasons? Adapter upgrades/replacement on a live system? I can imagine
someone using different HBAs so that they won't hit the same bug in both
HBAs, but that is a weak argument; I would think such systems would want
some type of cluster failover.

If the HBAs had the same memory and other limitations, it should function
OK, but it is hard to figure out exactly what might happen (if the HBAs had
different error handling characteristics, handled timeouts differently,
etc.). It would be easy to get rid of the checking for the same drivers,
(the code actually checks for the same drivers via Scsi_Host::hostt, not
the same hardware) - so it would allow multiple paths if the same driver
is used for different HBA's.

> And the Scsi_Device might be quite different. Imagine something like two
> storage boxes which do internal replication among them; yes, you'd only want
> to use one of them normal (because the Cache-coherency traffic is going to
> kill performance otherwise), but you can failover from one to the other even
> if they have different SCSI serials etc.

> And software RAID on top of multi-pathing is a typical example for a truely
> fault tolerant configuration.
> 
> Thats obviously easier with md, and I assume your SCSI code can also do that
> nicely.

I haven't tried it, but I see no reason why it would not work.

> > Agreed, but having the block layer be everything is also wrong.
> 
> Having the block device handling all block devices seems fairly reasonable to
> me.

Note that scsi uses the block device layer (the request_queue_t) for
character devices - look at st.c, sg.c, and sr*.c, calls to scsi_do_req()
or scsi_wait_req() queue to the request_queue_t. Weird but it works - you can
open a CD via sr and sg at the same time.

> > My view is that md/volume manager multi-pathing is useful with 2.4.x, scsi
> > layer multi-path for 2.5.x, and this (perhaps with DASD) could then evolve
> > into generic block level (or perhaps integrated with the device model)
> > multi-pathing support for use in 2.7.x. Do you agree or disagree with this
> > approach?
> 
> Well, I guess 2.5/2.6 will have all the different multi-path implementations
> mentioned so far (EVMS, LVM2, md, scsi, proprietary) - they all have code and
> a userbase... All of them and future implementations can benefit from better
> error handling and general cleanup, so that might be the best to do for now.
> 
> I think it is too soon to clean that up and consolidate the m-p approaches,
> but I think it really ought to be consolidated in 2.7, and this seems like a
> good time to start planning for that one.
> 
> 
> Sincerely,
>     Lars Marowsky-Br?e <lmb@suse.de>

The scsi multi-path code is not in 2.5.x, and I doubt it will be accepted
without the support of James and others.

-- Patrick Mansfield
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