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Date:      Fri, 29 Sep 2006 09:47:31 -0700
From:      Chris <snagit@cbpratt.prohosting.com>
To:        FreeBSD Questions <freebsd-questions@freebsd.org>
Subject:   Re: Swap Size Importance?
Message-ID:  <569ECAEE-4AEF-431B-B724-E734F8EE44E2@cbpratt.prohosting.com>
In-Reply-To: <20060929160631.GE657@gizmo.acns.msu.edu>
References:  <BAB82621-2B6E-4D53-9B0B-C7158E945E95@cbpratt.prohosting.com> <20060929160631.GE657@gizmo.acns.msu.edu>

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On Sep 29, 2006, at 9:06 AM, Jerry McAllister wrote:

> On Fri, Sep 29, 2006 at 08:52:58AM -0700, Chris wrote:
>
>>
>> Is there any shortfall in performance or reliability to running
>> production with swap equal in size to the 8gb of system memory? I
>
> doesn't matter much.   But, if you run enough to actually cause
> paging - which goes to swap space - then it becomes an issue.  Also,
>
I am assuming that real paging of active processes is death to that  
server anyway and means something else has to be throttled back with  
tuning of network bufs, apache or mysql. Same for crash dumps, can't  
run a server that is taking dumps or you lose your traffic.

> I think some things that get pulled to execute often can get left
> in swap space and accessed more quickly that all the way from main
> disk each time.  eg the system keeps track of what it has in swap and
> it is more efficient to read from swap - less overhead.   But someone

This is the part that concerned me. If one views a top on well  
running system and sees no swapping, I wanted to make certain there  
is no magic going on behind the scenes where processes have been  
mapped to swap in such a way that I could be currently benefitting  
from swap being higher than actual and not know it. If top is an  
accurate read on whether the system has placed high use processes in  
swap then it would suggest the first post is correct, and a memory  
rich system, where you configure to never exceed real memory, wastes  
that storage taken in swap. For expensive drives, given the sizes we  
use in RAM now, it's hard to justify. In the case of attempting this  
raid-5 configuration, it equates to the loss of 24G in scsi storage.  
I will run with 8g on the system drive.

Thank you very much for the responses.



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