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List:       opensolaris-storage-discuss
Subject:    Re: [storage-discuss] Re:  ssd driver tuning on sparc systems
From:       Ed Nadolski <Ed.Nadolski () Sun ! COM>
Date:       2006-09-28 13:29:09
Message-ID: 451BCE25.1000802 () sun ! com
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Vic Engle wrote:
> Hi James,
> 
> Thanks for the info. After having read about the tunable parameters my
impression was that the default values were designed to work in most
environments but in some specific environments they might not be optimal. My
thinking with the ssd_max_throttle was that path failover time would be a little
quicker if my host has to retry fewer commands so I was thinking that setting
max_throttle to as low a value as possible without affecting I/O performance on
my host would be desirable. Also, this would provide some protection to me
against having a bunch of my hosts fill up the target port queue on my array.

I want to put forth a caveat here: in general you should avoid modifying driver 
global variables except perhaps when you need to solve a very specific, 
nontrivial problem and have good reason to believe the change will work as you 
expect, and there is no other practical way to address the issue.

It isn't really right to think of driver global variables as "tunables" as if 
they are knobs that were meant to be set by the end user.  The globals that you 
mention here regulate the sd driver's timings and other behaviors under certain 
error conditions. The system isn't qualified with any other values than the 
defaults, but nondefault values can lead to unexpected behaviors/interactions 
that only occur in hard-to-reproduce cases, or with certain combinations of 
systems, transports, LUNs, etc.  Another issue is that driver globals are not a 
supported/stable interface, thus they can change names, values, and/or 
functionality (or even go away entirely) between releases or even in a patch.

I'm not looking to spread FUD here, but just be aware that this is a two-edged 
sword.  If you have an issue that can only be resolved that way, it's probably 
best to file a bug/RFE.

Regards,
Ed

-- 
Edmund Nadolski
Sun Microsystems Inc.
ed.nadolski@sun.com
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