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Don Burleson Blog 







Setting Server Swap Space for Oracle

Oracle Database Tips by Donald BurlesonApril 14, 2015


One important system administration task is allocating the "swap space" for an Oracle server.  In cases of servers with multi-speed RAM (Fast T1, located near the CPU), and slower T2 RAM, The UNIX/Linux system administrator will frequently allocate T1 RAM for the Oracle server.

An Oracle ACE notes:

For those of us who review the Oracle installation documentation, we know that Oracle suggests that you configure a substantial amount of swap space.

  • If you have between 1 and 2G RAM, you need to configure 1.5x RAM for swap space.  

  • For 2 to 8G RAM, swap space has to equal RAM.  

  • For RAM more than 8G, swap needs to be - RAM.

No doubt your system administrator (in Linux or Unix) will tell you that that is more swap than you need. He may tell you that you only need one or two gig of swap when you have plenty of memory.

Despite what the official Oracle documentation says, my recommendation is that the minimum swap space should be equal to the available RAM, and 2x swap to RAM is the maximum amount that should be needed.

I can tell you from personal experience, that your system administrator is wrong if he tells you that. I have been there, and done that as the saying goes. Make sure you configure the full amount of swap recommended by Oracle, more if possible. Here's why:

  • In a Unix operating system, there are two methods of allocating swap, eager swap and lazy swap. Under lazy swap, swap space is allocated only when needed, that is when an active process needs additional memory and all real memory is used up. If there is insufficient swap space available, the Operating system will create more swap space by killing idle processes. Obviously this is a dangerous and undesirable situation. 

  • Under Eager swap, as a process consumes memory, it also marks the same amount of swap space as used, so, for example, if your oracle database has 6G RAM allocated in the SGA, it will allocate both 6G RAM and 6G swap. If there is insufficient swap space, it won't kill other processes to make space. It will lock up the program attempting to allocate space, and eventually any other processes on the system that need swap space will also hang. In Solaris, I have seen such errors as ?unable to fork a process? or Solaris error 12, not enough space. In oracle, you will get messages like this: TNS-12500, TNS-12540, TNS-12560, TNS-00510. Usually your server will lock up shortly after the note enough space or unable to fork a process message.

In this example above, the TNS errors were my fault. I had given the Unix system administrator the requirements, including the amount of swap required. However, I had not followed up to make sure the settings were actually in effect. As it turns out, the Unix administrator had only allocated about 1/3 of the system RAM for swap, relying on his knowledge and understanding of swap.

This was my fault because as an Oracle DBA, it was my job to verify everything before you hand the installation over to the users. That includes the system level settings. If you can't see the settings (but as a DBA you should be able to see the settings), you have to rely on your system administrators word. But I could see the settings, I just didn't follow up and verify them.

Some Oracle DBA's I have talked to have allocated twice as much swap space as system RAM, following the older Oracle guidelines. After doing some researched I have discovered that Unix can be configured used to hold an image of the operating system processes for diagnostic purposes in the event of a system crash.

Reader feedback:

I'd like to reply. (6/25/2010)
Most sysadmins that I've met are as passionate about eking performance out of their systems as the DBA's.  In the best situations there's a meeting of the minds that leads to both harmony and performance.
Other times there's disagreement, generally with reasoned arguments on both sides.  In this case, I think the post referenced misses a couple of points.
I believe one should have a pretty good idea of how a system is going to be used before setting it up.  This implies that you have an idea of how many users, how much bandwidth, disk space, memory, and CPU are going to be required in order to for the system to do the tasks assigned.  If you're configuring based on guesswork, then you should expect problems.  In the bad old days when systems didn't support much memory and it was very expensive, using more swap could make some economic sense.
If you are configuring based upon an understanding of the requirements, then (assuming you have the resources to buy what you need) you have already decided how much memory is needed in order to do the job.  In this case, unless there is some reason why you want to slow the system down by knowingly setting it up to do lots of paging, you then have to determine the relationship between swap disk and memory in your particular case.
Most modern *nix operating systems either default to or can be configured to use lazy swap, and even most of those that use eager swap don't require a 1-1 match for RAM.  Given this, my contention is that if you have configured the server properly (acquired the proper model and populated it with the proper amount of RAM) then you just need enough to cover unanticipated shortfalls due to unusual situations. 

If you haven't configured enough physical memory, then configure more!  In today's systems and at today's memory prices, swap disk should not be used as a substitute for adequate RAM.  Besides that, you can get out-of-space errors even with more swap configured if you didn't work out the system configuration properly to begin with.
Sometimes there are cases where you're stuck with something older (HW and/or OS) or more limited, that isn't optimal for your use.  If this is the case, please be sure to discuss the situation with the administrator -- this is a different case.


Related on on server swap allocation for Oracle

See my related notes on configuring server swap space:


If you like Oracle tuning, you might enjoy my book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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