Legends don't die or fade away. Not even the ones about SAP and pagefiles
One of the most heard things about SAP and hardware resources is that you need at least twice the amount of RAM configured as swap or as a paging file. Sometimes you will even hear that you need up to three of four times the RAM.
Now, this was actually true.... In 2003 or so.
Nonetheless I still receive mails about this recommendation. One that came in today was something along these lines:
Now, this would still be ok if we were talking about physical machines that are relatively small. If there wasn't the fact that we also have servers that are quite a bit bigger and carry 256GB of RAM on them. Configuring 2x the amount RAM or even 4x that amount would just be a waste of disk space, and to get this part written down:The SAP installation guide recommendations that the amount of swap space on the server be twice the amount of RAM - this recommendation is valid for Database, Central Instance, and DIA instances.
The following servers all have 32GB of RAM allocated to them. They also all have 32GB of swap configured. In order to comply with the prerequisites we'll need to increase the swap space on each to 64GB as soon as possible.
I'ts no longer required on IA64 and x64!
People wrote down the numbers of some SAP notes once, and they continue to refer to these same notes instead of just checking if there perhaps is an updated version of it flying around somewhere.
Now this could spark up an entirely different discussion about versioning of documents and the ability to locally store copies of documents. Anyone would have a blast at an ISO 20000 audit.
Anyway, to get rid of the rumors in this post, at this point in time you can check the following SAP note #146289 (link may require a valid service marketplace account) which in a nutshell states the following:
And this is valid for all installations with a 64-bit kernel. If you want to print it, add a date of the last change, or just subscribe to the document, and remember to perhaps check after a year or so if some newer info is available. After all we are working in the IT-business and things do tend to change every now and then.When implementing the 64-bit kernel, we recommend at least
20 GByte of swap space. (Plus approx.10 GB for every additional 64-bit instance on the same computer) See note 153641 for more information.
In a previous life my basis team implemented the x64 kernel and found that with a patch it resolved excessive volumes of paging.
Shouldnt paging be only used if you cannot facilitate on the physical memory?
Setting up the pagefile is a matter for a systems administrator, who should choose just how big the file should be. Applications should not interfere with this process; after all, systems whose physical memory exceeds the sum of RAM requirements and swap space requirements should be perfectly capable of running a certain application, without even HAVING a pagefile.
I've been running my desktop without a pagefile for over a year, now, maybe even more than two years. It's been running just fine. While I'm not making the claim that disabling your pagefile increases performance, it does (quite obviously) lower disk accesses. (At least in the default configuration of Windows XP Home. I'm not talking about Linux setups where the swappiness variable is set very low.)
P.s. Renegade, could you please fix my SAP system, it says as usual something about a spool log file full
Seriously, sometimes you want to hug the SAP Basis guys and sometimes you want to beat them
[quote(You can compare the costs for each GB of disk space (about $100.00) with consulting and support costs for memory problems)[/quote]
If you have a system capable of and running with 256GB of memory, who cares about couple of hundred GB for a pagefile?
To put this all in perspective: You're better of searching for a solution for the problem (why is my system swapping this much?) than easing blindly it's symptoms (enlarging pagefile to ~2,5 times RAaM.)
For people with Server 2003 w/lots of paging, please check Note 1009297...
@Tim: the SAP basis consultant gives a list of requirements to the system administrator. If the system administrator thinks he knows best, impements a 1GB pagefile and goes on holiday, while the ERP system comes to a screechy halt within seconds, guess whose head will come of after holiday.....
I don't think configuring a Windows system to use a smaller pagefile than the amount of installed physical memory is a good idea, and if the sysadmin is worth his salt, he likely wouldn't do such a thing in the first place. If a sysadmin is being an idiot, you've got much bigger problems to worry about.
In a Linux environment, doing this should not pose any problems, provided that the configuration (swappiness, mostly) is done properly.
I still consider it a bad thing when an application places demands on the availability of swap space. It's an outdated and downright silly thing to do so. It's bad practice. If you guys can show me a reason why an application should place requirements rather than recommendations on these things, please, do enlighten me. But so far, I have yet to hear a valid argument in favor of pagefile requirements, rather than mere recommendations.