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Old 04-24-2008, 08:45 PM
"Damon L. Chesser"
 
Default OT (slightly) swap limits

Being in an interview loop looking for employment, I find I am asked
questions I never considered, for example: How much is enough swap?


Simple, right? Try answering it. Used to be the limit was 2G
files/partitions with up to 8 partitions total. This does not apply
anymore: I have seen customers with 32G of swap.


Not wanting to talk about what they did with 32G of swap, but I am
trying to find documentation for the limits and the rules now.


For example: 2xRAM (or 1.5) is not considered valid any more is it? If
you have 64G of ram, you need 128G of swap. But in truth I don't know
and I am not finding much on google (lots covering the 2G OLD limit).
What I want is documentation so I am not working off of opinion (read
religious belief).


Got any links you want to share?

--
Damon L. Chesser
damon@damtek.com
http://www.linkedin.com/in/dchesser


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Old 04-24-2008, 09:00 PM
Patrick Ouellette
 
Default OT (slightly) swap limits

On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote:
>
> Being in an interview loop looking for employment, I find I am asked
> questions I never considered, for example: How much is enough swap?
>

Rephrase the question. Ask what the intended use of the machine is,
what response times the users expect, how much real RAM is there, and
what applications/services will run off the machine.

Then tell the interviewer how each parameter you've asked about would
influence your decision on how much swap was enough.

How much is enough? As much as the system needs to run and not kill
processes due to lack of memory (real + swap).

I've run machines with 1Gig or more RAM with NO SWAP. I've also run
machines with 4Gig of RAM and 16Gig of swap (BIG datasets).

Pat
--

Patrick Ouellette pat@flying-gecko.net
ne4po (at) arrl (dot) net Amateur Radio: NE4PO
"Crank the amp to 11, this needs more cowbell - and a llama wouldn't hurt either"
"Your arguments are an odd mix of overly optimistic on one side and overly
pessimistic on the other"


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Old 04-24-2008, 09:16 PM
"Damon L. Chesser"
 
Default OT (slightly) swap limits

Patrick Ouellette wrote:

On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote:

Being in an interview loop looking for employment, I find I am asked
questions I never considered, for example: How much is enough swap?





Rephrase the question. Ask what the intended use of the machine is,
what response times the users expect, how much real RAM is there, and
what applications/services will run off the machine.

Then tell the interviewer how each parameter you've asked about would
influence your decision on how much swap was enough.

How much is enough? As much as the system needs to run and not kill
processes due to lack of memory (real + swap).


I've run machines with 1Gig or more RAM with NO SWAP. I've also run
machines with 4Gig of RAM and 16Gig of swap (BIG datasets).

Pat


Well,

I tend to agree with you, however, I am being sucked dry of my Linux
knowledge (the purpose of the interview, find the point of breakdown to
determine the extent of the knowledge/skill). And much to my surprise I
just found this:


"At a bare minimum, you need an appropriately-sized root partition, and
a swap partition equal to twice the amount of RAM"

page 59, from a questionable source: Installation Guide of RHEL5.

and on page 62, they further detail it:

"Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and
then an additional

1x physical RAM for any amount above 2 GB, but never less than 32 MB.
So, if:
M = Amount of RAM in GB, and S = Amount of swap in GB, then
If M < 2
S = M *2
Else
S = M + 2
Using this formula, a system with 2 GB of physical RAM would have 4 GB
of swap, while one
with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap
space partition

can be especially helpful if you plan to upgrade your RAM at a later time.
For systems with really large amounts of RAM (more than 32 GB) you can
likely get away

with a smaller swap partition (around 1x, or less, of physical RAM)."

That is RHEL's take on the issue. Still looking for other sources.
Interesting, I think.



--
Damon L. Chesser
damon@damtek.com
http://www.linkedin.com/in/dchesser


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Old 04-24-2008, 09:26 PM
"Damon L. Chesser"
 
Default OT (slightly) swap limits

Damon L. Chesser wrote:
Being in an interview loop looking for employment, I find I am asked
questions I never considered, for example: How much is enough swap?


Simple, right? Try answering it. Used to be the limit was 2G
files/partitions with up to 8 partitions total. This does not apply
anymore: I have seen customers with 32G of swap.


Not wanting to talk about what they did with 32G of swap, but I am
trying to find documentation for the limits and the rules now.


For example: 2xRAM (or 1.5) is not considered valid any more is it?
If you have 64G of ram, you need 128G of swap. But in truth I don't
know and I am not finding much on google (lots covering the 2G OLD
limit). What I want is documentation so I am not working off of
opinion (read religious belief).


Got any links you want to share?

And because I think this is academically interesting I will post this
link here as well:


http://www.debian-administration.org/articles/550

interesting article on making a dynamic swap file at boot time that will
be twice your RAM size automagicaly.


--
Damon L. Chesser
damon@damtek.com
http://www.linkedin.com/in/dchesser


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Old 04-24-2008, 11:42 PM
Alex Samad
 
Default OT (slightly) swap limits

On Thu, Apr 24, 2008 at 05:16:56PM -0400, Damon L. Chesser wrote:
> Patrick Ouellette wrote:
>> On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote:
>>
>>> Being in an interview loop looking for employment, I find I am asked
>>> questions I never considered, for example: How much is enough swap?
>>>
>>>
>>
>> Rephrase the question. Ask what the intended use of the machine is,
>> what response times the users expect, how much real RAM is there, and
>> what applications/services will run off the machine.
>>
>> Then tell the interviewer how each parameter you've asked about would
>> influence your decision on how much swap was enough.
>>
>> How much is enough? As much as the system needs to run and not kill
>> processes due to lack of memory (real + swap).
>>
>> I've run machines with 1Gig or more RAM with NO SWAP. I've also run
>> machines with 4Gig of RAM and 16Gig of swap (BIG datasets).
>>
>> Pat
>>
> Well,
>
> I tend to agree with you, however, I am being sucked dry of my Linux
> knowledge (the purpose of the interview, find the point of breakdown to
> determine the extent of the knowledge/skill). And much to my surprise I
> just found this:
>
> "At a bare minimum, you need an appropriately-sized root partition, and
> a swap partition equal to twice the amount of RAM"
>
> page 59, from a questionable source: Installation Guide of RHEL5.

interestingly RHEL4 at 2.6 kernel distro still also made swap files of
2G max and used multiples of that. Since 2.6 this hasn't been
necessary.

Just because RHEL does it that way doesn't make it right

Patrick I believe has the better approach - what are you going to use
the box for and what sort of response do you want, although I have to
diss agree on the need for large swap space for a database, DB are
engineered to use all the space that are told to get hold of and have
their own caching. Why interfere with it by pretending you have more
memory than you do. You could end up hitting swap because the DB cache
has grown.



>
> and on page 62, they further detail it:
>
> "Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and
> then an additional
> 1x physical RAM for any amount above 2 GB, but never less than 32 MB.
> So, if:
> M = Amount of RAM in GB, and S = Amount of swap in GB, then
> If M < 2
> S = M *2
> Else
> S = M + 2
> Using this formula, a system with 2 GB of physical RAM would have 4 GB
> of swap, while one
> with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap
> space partition
> can be especially helpful if you plan to upgrade your RAM at a later time.
> For systems with really large amounts of RAM (more than 32 GB) you can
> likely get away
> with a smaller swap partition (around 1x, or less, of physical RAM)."
>
> That is RHEL's take on the issue. Still looking for other sources.
> Interesting, I think.
>
>
> --
> Damon L. Chesser
> damon@damtek.com
> http://www.linkedin.com/in/dchesser
>
>
> --
> To UNSUBSCRIBE, email to debian-user-REQUEST@lists.debian.org with a
> subject of "unsubscribe". Trouble? Contact listmaster@lists.debian.org
>
>

--
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- George W. Bush
03/29/2001
Washington, DC
White House press conference,
 
Old 04-25-2008, 04:14 AM
Kamaraju S Kusumanchi
 
Default OT (slightly) swap limits

Damon L. Chesser wrote:

> Being in an interview loop looking for employment, I find I am asked
> questions I never considered, for example: How much is enough swap?
>

For desktops, from my experience, I would say you need at least as much as
your RAM. To be on the safe side, I go for 2 times the RAM.

I hibernate my desktops pretty frequently. When hibernating a machine, a
snapshot of the RAM is written on to the swap partition. That is why I
follow "at least as much as RAM" rule.

hth
raju
--
Kamaraju S Kusumanchi
http://www.people.cornell.edu/pages/kk288/
http://malayamaarutham.blogspot.com/


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Old 04-25-2008, 04:40 AM
Thierry Chatelet
 
Default OT (slightly) swap limits

On Thursday 24 April 2008 22:45:05 Damon L. Chesser wrote:
> Being in an interview loop looking for employment, I find I am asked
> questions I never considered, for example: How much is enough swap?
>
> Simple, right? Try answering it. Used to be the limit was 2G
> files/partitions with up to 8 partitions total. This does not apply
> anymore: I have seen customers with 32G of swap.
>
> Not wanting to talk about what they did with 32G of swap, but I am
> trying to find documentation for the limits and the rules now.
>
> For example: 2xRAM (or 1.5) is not considered valid any more is it? If
> you have 64G of ram, you need 128G of swap. But in truth I don't know
> and I am not finding much on google (lots covering the 2G OLD limit).
> What I want is documentation so I am not working off of opinion (read
> religious belief).
>
> Got any links you want to share?
>
> --
> Damon L. Chesser
> damon@damtek.com
> http://www.linkedin.com/in/dchesser

Another idea would be to ask the question on the d-i mailing list. I noticed
that, if you choose to let it do an automatic partitioning of the disk, it
will size the swap partition according to ram and not to disk size. So,
someone there must know the magic formula.
Thierry


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Old 04-25-2008, 05:51 AM
Celejar
 
Default OT (slightly) swap limits

On Fri, 25 Apr 2008 06:40:25 +0200
Thierry Chatelet <tchatelet@free.fr> wrote:

[snip]

> Another idea would be to ask the question on the d-i mailing list. I noticed
> that, if you choose to let it do an automatic partitioning of the disk, it
> will size the swap partition according to ram and not to disk size. So,
> someone there must know the magic formula.

Frans Pop (of the Install Team) told me that:

> Guided partitioning sets an upper limit of 300% memory size for swap
> partitions. It is true that that is somewhat high for system with a lot
> of internal memory. However, it does not do any harm either.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=410328

> Thierry

Celejar
--
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ssuds.sourceforge.net - A Simple Sudoku Solver and Generator


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Old 04-25-2008, 12:50 PM
"Damon L. Chesser"
 
Default OT (slightly) swap limits

Damon L. Chesser wrote:
Being in an interview loop looking for employment, I find I am asked
questions I never considered, for example: How much is enough swap?


Simple, right? Try answering it. Used to be the limit was 2G
files/partitions with up to 8 partitions total. This does not apply
anymore: I have seen customers with 32G of swap.


Not wanting to talk about what they did with 32G of swap, but I am
trying to find documentation for the limits and the rules now.


For example: 2xRAM (or 1.5) is not considered valid any more is it?
If you have 64G of ram, you need 128G of swap. But in truth I don't
know and I am not finding much on google (lots covering the 2G OLD
limit). What I want is documentation so I am not working off of
opinion (read religious belief).


Got any links you want to share?


Well,

it is nice to know I was not nuts in my answer to their question. I
thought perhaps technology moved ahead of me or I forgot something and I
did not get the memo. But to be more clear, I am talking about
servers. Anybody do anything different with servers or do we still
stick with the 1.5/2X rule or perhaps the RH S=M+1 for >2G rule (with
the nice stipulation that perhaps for machines with really large amounts
of RAM you MIGHT use less)?


--
Damon L. Chesser
damon@damtek.com
http://www.linkedin.com/in/dchesser


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Old 04-25-2008, 03:26 PM
Patrick Ouellette
 
Default OT (slightly) swap limits

On Fri, Apr 25, 2008 at 09:42:03AM +1000, Alex Samad wrote:
>
> On Thu, Apr 24, 2008 at 05:16:56PM -0400, Damon L. Chesser wrote:
> > Patrick Ouellette wrote:
> >>
> >> I've run machines with 1Gig or more RAM with NO SWAP. I've also run
> >> machines with 4Gig of RAM and 16Gig of swap (BIG datasets).
> >>
> >> Pat
> >>
> > Well,
> >
> > I tend to agree with you, however, I am being sucked dry of my Linux
> > knowledge (the purpose of the interview, find the point of breakdown to
> > determine the extent of the knowledge/skill). And much to my surprise I
> > just found this:
> >
> > "At a bare minimum, you need an appropriately-sized root partition, and
> > a swap partition equal to twice the amount of RAM"
> >
> > page 59, from a questionable source: Installation Guide of RHEL5.
>
> interestingly RHEL4 at 2.6 kernel distro still also made swap files of
> 2G max and used multiples of that. Since 2.6 this hasn't been
> necessary.
>
> Just because RHEL does it that way doesn't make it right
>
> Patrick I believe has the better approach - what are you going to use
> the box for and what sort of response do you want, although I have to
> diss agree on the need for large swap space for a database, DB are
> engineered to use all the space that are told to get hold of and have
> their own caching. Why interfere with it by pretending you have more
> memory than you do. You could end up hitting swap because the DB cache
> has grown.
>

I just want to clarify. I said there were big datasets, not necessarily
a database application.

Pat

--

Patrick Ouellette pat@flying-gecko.net
ne4po (at) arrl (dot) net Amateur Radio: NE4PO
"Crank the amp to 11, this needs more cowbell - and a llama wouldn't hurt either"
"Your arguments are an odd mix of overly optimistic on one side and overly
pessimistic on the other"


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