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Full Discussion: Please advise good source of info about swapping
Top Forums UNIX for Advanced & Expert Users Please advise good source of info about swapping Post 302614425 by methyl on Wednesday 28th of March 2012 01:12:31 PM
Old 03-28-2012
1) The O/S will do necessary paging as processes are created and die. If you have no swap, memory will become fragmented.

2) You should never set the swap size to zero because the O/S still needs to shuffle pages around in the background. While you are tuning Oracle you need substantial leeway and I would have swap at least the same size as memory to allow for future expansion, unexpected peaks and configuration mistakes. I'd rather have the system run slow than crash.

Oracle makes extensive use of O/S Shared Memory and you must allow memory for this and build the kernel correctly.

Most unix O/S dedicate 25% of memory to disc buffers by default. You either allow for it or change the kernel parameters. If the system only ever runs Oracle then there is much scope to give memory to Oracle disc buffers.
Many Oracle DBAs don't allocate enough memory for table sorts in the SGA.

There are huge and comprehensive manuals from Oracle regarding sizing and tuning on each different O/S. In my experience Oracle will never run properly straight out of the box.

3) Yes if you have 8Gb RAM and 8Gb swap the most virtual memory you could ever use is 16Gb.
There was a historic requirement to have a minimum swap of the same size as memory and older unix would not let you use any physical memory above that point. There was no mirroring, but swap was also used for dump in the event of a system crash and it had to be big enough. That rule has been relaxed on some modern O/S.
Older unix did indeed recommend creating swap at 2x or even 3x memory and swap was used for storing idle processes on heavily loaded systems (but it was slow if any active processes got displaced into swap).
Last edited by methyl; 03-28-2012 at 02:22 PM..
 

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LEARN ABOUT HPUX

remote_nfs_swap

remote_nfs_swap(5)						File Formats Manual						remote_nfs_swap(5)

NAME

       remote_nfs_swap - enable swapping across NFS

VALUES

   Default
   Allowed values
DESCRIPTION

       This  tunable  controls adding a NFS filesystem for use as swap.  If is set to only local filesystems and devices can be used for swap.	If
       it is set to both local and networked file systems can be used for swap.

       Historically, this tunable was used in NFS clusters that are no longer supported, but this capability has not yet been removed.

   Who Is Expected to Change This Tunable?
       Anyone.

   Restrictions on Changing
       Changes to this tunable take effect on the next boot.

   When Should the Tunable Be Turned On?
       Almost never.  As previously mentioned, this tunable was designed for systems which are not in use  today.   Only  systems  with  extremely
       robust NFS capabilities should even consider using NFS for swap.

   What Are the Side Effects of Turning the Tunable On?
       If  the	value  is  set to and a NFS partition is added as swap, then some kernel memory will be set aside for NFS transactions to the swap
       file system.  The kernel will then use the NFS swap partition just the same as a local file system.  If the NFS capabilities of the  system
       are  not  robust,  this	could lead to extremely long swap times (both swap in and swap out), and possibly a loss of memory because all the
       reserved memory and more could be used by NFS (just when memory pressure is high) causing the need for swap.

   When Should the Tunable Be Turned Off?
       Unless you are positive that the NFS system can handle the load of being used as swap, and does not have a local file system or disk  drive
       to spare, this tunable should be set to

   What Are the Side Effects of Turning the Tunable Off?
       Any  previously	defined  NFS  swap  filesystems will no longer be allowed.  Kernel memory will not be reserved for NFS transactions due to
       swap.

   What Other Tunables Should Be Changed at the Same Time?
       None.

WARNINGS

       All HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases	of
       HP-UX.

       Installation  of  optional  kernel  software, from HP or other vendors, may cause changes to tunable parameter values.  After installation,
       some tunable parameters may no longer be at the default or recommended values.  For information about the effects of installation  on  tun-
       able  values,  consult  the documentation for the kernel software being installed.  For information about optional kernel software that was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

							     Tunable Kernel Parameters						remote_nfs_swap(5)
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