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Kaushal Shriyan 06-18-2012 06:02 PM

Swap Partition in CentOS 5.8
 
Hi,

Any recommendation to set SWAP Partition in CentOS 5.8 on 32GB Physical
Memory Box and also how does swap partition is being used and any utility
to know when was swap partition being used.

Regards,

Kaushal
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06-18-2012 07:19 PM

Swap Partition in CentOS 5.8
 
Kaushal Shriyan wrote:
>
> Any recommendation to set SWAP Partition in CentOS 5.8 on 32GB Physical
> Memory Box and also how does swap partition is being used and any utility
> to know when was swap partition being used.

In the Days of Yore, received wisdom was 2-2.5 times RAM. For the last
five or so years, with the massive growth of RAM, it's been 2G for pretty
much everything. The system will use it mostly as a last resort, if it's
being overwhelmed.

mark

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Les Mikesell 06-18-2012 07:33 PM

Swap Partition in CentOS 5.8
 
On Mon, Jun 18, 2012 at 2:19 PM, <m.roth@5-cent.us> wrote:
> Kaushal Shriyan wrote:
>>
>> Any recommendation to set SWAP Partition in CentOS 5.8 on 32GB Physical
>> Memory Box and also how does swap partition is being used and any utility
>> to know when was swap partition being used.
>
> In the Days of Yore, received wisdom was 2-2.5 times RAM. For the last
> five or so years, with the massive growth of RAM, it's been 2G for pretty
> much everything. The system will use it mostly as a last resort, if it's
> being overwhelmed.

Yes, if you actually need/use more than 2G swap, the system will
likely be so unresponsive that you'll think it is dead and reboot
anyway. As for seeing if it is used, 'top' and 'free' will show it,
or you can track activity with vmstat.

--
Les Mikesell
lesmikesell@gmail.com
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Christina Salls 06-18-2012 10:30 PM

Swap Partition in CentOS 5.8
 
This is actually from RHEL admin guide but should be appropriate for CentOS
as well:

Table 6.1. Recommended System Swap Space
Amount of RAM in the SystemRecommended Amount of Swap Space4GB of RAM or
lessa minimum of 2GB of swap space4GB to 16GB of RAMa minimum of 4GB of
swap space16GB to 64GB of RAMa minimum of 8GB of swap space64GB to 256GB of
RAMa minimum of 16GB of swap space256GB to 512GB of RAMa minimum of 32GB of
swap space


>


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Woodchuck 06-18-2012 11:55 PM

Swap Partition in CentOS 5.8
 
On Mon, Jun 18, 2012 at 06:30:28PM -0400, Christina Salls wrote:
> This is actually from RHEL admin guide but should be appropriate for CentOS
> as well:
>

Let me un-windoze your table: (you should consider using a text-based
mail agent on Unix lists).

<PRE>

Table 6.1. Recommended System Swap Space

Amount of RAM in the System Recommended Amount of Swap Space
4GB of RAM or less a minimum of 2GB of swap space
4GB to 16GB of RAM a minimum of 4GB of swap space
16GB to 64GB of RAM a minimum of 8GB of swap space
64GB to 256GB of RAM a minimum of 16GB of swap space
256GB to 512GB of RAM a minimum of 32GB of swap space

</PRE>

The problem with this -- it seems to assume that swap requirements
are based on system memory. I'm not intimately familiar with the
Linux kernel, so I have a few questions: (These are T-F).

1) The linux kernel does not swap at all, but only pages. (Swap ==
copy entire image to disc, and suspend it. Page == copy certain
memory pages to disc when core becomes exhausted.)

2) The linux kernel does not copy the entire image to the page file
to start execution. (This was done on some older OSes, one copied
the whole image to swap/page, then faulted the page with the entry
point, which got the ball rolling. Initially this was a copy to
"drum" from tape or cards, but there was for a long time situations
with slow vs fast storage.) Instead, pages are initially paged in
from the regular normal disc image of the program or from dynamic load
libraries.

3) The practical upper limit on swap [sic] file size are the page
maps in the kernel, (which map pages of VM to blocks in the swap file),
since these maps (naturally) cannot be paged.

4) Linux kernel memory may not be paged, any of it, ever. (This
is true of some other systems.)

5) The amount of swap/page required in a system is a function of
the number and size of (pseudo)-simultaneously executing images.

6) The recommendations in the table are ad hoc estimates based
on questions (5) and (3) being answered "TRUE".

7) Memory-mapping a disc file with mmap(2) places no demand on
system page files.

8) The Linux kernel itself has no requirement at all for a page
file or partition.

Thanks to the list for any answers!

Dave
--
Aristippus passed Diogenes as he was washing lentils. He said,
"If you could but learn to flatter the king, you would not have
to live on lentils." Diogenes said, "And if you could learn to
live on lentils, you would not have to flatter the king."
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Nate Duehr 06-20-2012 05:44 PM

Swap Partition in CentOS 5.8
 
On Jun 18, 2012, at 5:55 PM, Woodchuck wrote:

[snipped swap/pagefile list of statements]

> Thanks to the list for any answers!


Nothing in your list was phrased as a question. What are you trying to determine? If the list of statements was accurate?

Nate
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