Date: Tue, 30 Mar 2004 15:12:36 -0700
From: "Justin T. Gibbs" <gibbs@scsiguy.com>
Reply-To: "Justin T. Gibbs" <gibbs@scsiguy.com>
To: Jeff Garzik <jgarzik@pobox.com>
cc: Kevin Corry <kevcorry@us.ibm.com>, linux-kernel@vger.kernel.org,
       Neil Brown <neilb@cse.unsw.edu.au>, linux-raid@vger.kernel.org,
       dm-devel@redhat.com
Subject: Re: "Enhanced" MD code avaible for review
Message-ID: <1001500000.1080684755@aslan.btc.adaptec.com>
In-Reply-To: <4069EB03.9000202@pobox.com>
References: <760890000.1079727553@aslan.btc.adaptec.com> <200403261315.20213.kevcorry@us.ibm.com> <1644340000.1080333901@aslan.btc.adaptec.com> <200403270939.29164.kevcorry@us.ibm.com> <842610000.1080666235@aslan.btc.adaptec.com> <4069AB1B.90108@pobox.com> <854630000.1080668158@aslan.btc.adaptec.com> <4069B289.9030807@pobox.com> <866290000.1080669880@aslan.btc.adaptec.com> <4069EB03.9000202@pobox.com>
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>> So you are saying that this presents an unrecoverable situation?
> 
> No, I'm saying that the data phase need not have a bunch of in-kernel
> checks, it should be generated correctly from the source.

The SCSI drivers validate the controller's data phase based on the
expected phase presented to them from an upper layer.  I never talked
about adding checks that make little sense or are overly expensive.  You
seem to equate validation with huge expense.  That is just not the
general case.

>> Hmm.  I've never had someone tell me that my SCSI drivers are slow.
> 
> This would be noticed in the CPU utilization area.  Your drivers are
> probably a long way from being CPU-bound.

I very much doubt that.  There are perhaps four or five tests in the
I/O path where some value already in a cache line that has to be accessed
anyway is compared against a constant.  We're talking about something
down in the noise of any type of profiling you could perform.  As I said,
validation makes sense where there is basically no-cost to do it.

>> I don't think that your statement is true in the general case.  My
>> belief is that validation should occur where it is cheap and efficient
>> to do so.  More expensive checks should be pushed into diagnostic code
>> that is disabled by default, but the code *should be there*.  In any event,
>> for RAID meta-data, we're talking about code that is *not* in the common
>> or time critical path of the kernel.  A few dozen lines of validation code
>> there has almost no impact on the size of the kernel and yields huge
>> benefits for debugging and maintaining the code.  This is even more
>> the case in Linux the end user is often your test lab.
> 
> It doesn't scale terribly well, because the checks themselves become a
> source of bugs.

So now the complaint is that validation code is somehow harder to write
and maintain than the rest of the code?

--
Justin

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