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Full Discussion: Threshold for swap memory
Top Forums UNIX for Dummies Questions & Answers Threshold for swap memory Post 302609717 by vbe on Tuesday 20th of March 2012 10:48:28 AM
Old 03-20-2012
I think the best way to answer your concern is:
Should a system swap? Only when under heavy usage?
- This should be episodic...If it more than weekly, then consider investing in more RAM.

Can the way swap is configured have an impact on performance when swapping?
- Yes, device swap is preferable, only use file swap when can not do anything else...
Then, if more than on disk, swap between all disks is far better than one big one on OS disk... e.g. Two 1 GB swaps on 2 disks is better than 2GB on only one disk
I had a system that always used swap (7 oracle instances running plus 1 sybase..) because it had only 2GB RAM since at the time it costed $$$$...
It had permanently 500MB of active swap with un-noticeable performance impact...
It had 4X1GB swap on 4 separate arrays on 2 separate contollers...

If you create 2 swaps to optimize swap then you must make them same size, in which case it will use both equally... otherwise the biggest is used first...
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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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