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Full Discussion: What swap is my system running on?
Top Forums UNIX for Advanced & Expert Users What swap is my system running on? Post 302070111 by Perderabo on Friday 31st of March 2006 01:08:01 PM
Old 03-31-2006
Swap is only loosely associated with physical memory. You would be better off forgeting about physical memory while you look at those numbers. Those swap areas that you know about on disk...they add up to your total swap space. Some of your total swap space may be in use...if so, this is known as "used swap space". But you you may have some swap space not yet in use...if so, this is called "available swap space". The "used swap space" may actually have stuff written to it...that is "allocated swap space". The kernel know exactly where the allocated swap space is. There may be some stuff in physical memory that has never been swapped out, but is potentially swapable...some swap space has been "reserved" for this stuff. The kernel only has a counter for "reserved swap space". If and when "reserved swap space" is needed it will become "allocated swap space". Allocation takes some time and kernel delays it. Lots of stuff come into core and never really need to swap. So no allocation will ever be done.

You might have swap space equal to 3 times physical memory. And you can potentially run out of swap anyway.
 

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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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