Swap Size Importance?

As a standard practice, I've always configured swap file to
be double  
the size of real ram split across system and data disk. For
example,  
8gb on da0 and 8gb on da1 if the system has 8g real ram. In
practice,  
In 7 or 8 years, I've never seen swap used for anything but
a few k  
of inactive processes and I would imagine if real active
process  
swapping occurred, it would be an immediate indicator that
the system  
that isn't responsive enough for use anymore and requires
upgrade or  
tuning. Can't run a website process off disk and keep anyone
coming  
to the site . (BTW,
I'm talking only about high end servers, not  
test boxes where I've seen lots of swapping).

I'm at the point of attempting my first gvinum software
raid-5 and  
realized, I need the entire disk storage of all three
non-system  
drives to avoid pulling an 8gb chunk out of the drive sizes.
The  
configuration is one scsi 72g system disk and 3 that will be
used for  
the raid volume. I should mention I turn off dumps, haven't
found the  
use for that in a production server since it should not be
rebooting  
or it's back in the shop and another box is taking it's
place.

Is there any shortfall in performance or reliability to
running  
production with swap equal in size to the 8gb of system
memory? I  
can't think of any but don't want to make a hard to correct
mistake  
once this thing goes in. 
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On Friday 29 September 2006 11:52, Chris wrote:
> As a standard practice, I've always configured swap
file to be double
> the size of real ram split across system and data disk.
For example,
> 8gb on da0 and 8gb on da1 if the system has 8g real
ram. In practice,
> In 7 or 8 years, I've never seen swap used for anything
but a few k
> of inactive processes and I would imagine if real
active process
> swapping occurred, it would be an immediate indicator
that the system
> that isn't responsive enough for use anymore and
requires upgrade or
> tuning. Can't run a website process off disk and keep
anyone coming
> to the site . (BTW,
I'm talking only about high end servers, not
> test boxes where I've seen lots of swapping).
>
> I'm at the point of attempting my first gvinum software
raid-5 and
> realized, I need the entire disk storage of all three
non-system
> drives to avoid pulling an 8gb chunk out of the drive
sizes. The
> configuration is one scsi 72g system disk and 3 that
will be used for
> the raid volume. I should mention I turn off dumps,
haven't found the
> use for that in a production server since it should not
be rebooting
> or it's back in the shop and another box is taking it's
place.
>
> Is there any shortfall in performance or reliability to
running
> production with swap equal in size to the 8gb of system
memory? I
> can't think of any but don't want to make a hard to
correct mistake
> once this thing goes in.

Nope. I routinely run boxes with 512MB or 1GB of swap, even
if the RAM size is 
much higher than that. You won't have anywhere to save a
crashdump in that 
case, but you seem to already be aware of that.

JN
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On Fri, Sep 29, 2006 at 08:52:58AM -0700, Chris wrote:

> As a standard practice, I've always configured swap
file to be double  
> the size of real ram split across system and data disk.
For example,  
> 8gb on da0 and 8gb on da1 if the system has 8g real
ram. In practice,  
> In 7 or 8 years, I've never seen swap used for anything
but a few k  
> of inactive processes and I would imagine if real
active process  
> swapping occurred, it would be an immediate indicator
that the system  
> that isn't responsive enough for use anymore and
requires upgrade or  
> tuning. Can't run a website process off disk and keep
anyone coming  
> to the site . (BTW,
I'm talking only about high end servers, not  
> test boxes where I've seen lots of swapping).
> 
> I'm at the point of attempting my first gvinum software
raid-5 and  
> realized, I need the entire disk storage of all three
non-system  
> drives to avoid pulling an 8gb chunk out of the drive
sizes. The  
> configuration is one scsi 72g system disk and 3 that
will be used for  
> the raid volume. I should mention I turn off dumps,
haven't found the  
> use for that in a production server since it should not
be rebooting  
> or it's back in the shop and another box is taking it's
place.
> 
> Is there any shortfall in performance or reliability to
running  
> production with swap equal in size to the 8gb of system
memory? I  
> can't think of any but don't want to make a hard to
correct mistake  
> once this thing goes in. 

It really depends on the number and size of processes you
will be
running.  It you have a large memory and generally run a mix
of
processes that will totally fit in memory, then it probably
doesn't
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 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 
else should know more about that than I.

////jerry

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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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