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RAM page fencing for Oracle

As we know, in a shared memory environment such as UNIX, a “virtual” memory operating system will allow for the referencing of RAM memory beyond the physical limit of RAM. When the physical RAM is exceeded, UNIX will “swap” the contents of RAM memory to the swap disk, and give the RAM memory to another task. This swapping is done in UNIX based upon a least-recently-used algorithm.

To see if your server is experiencing a shortage of RAM, you can view memory swapping on UNIX by starting the “vmstat” command, and looking at the page-in (pi) column. Whenever pi is non-zero, memory paging is occurring and Oracle may run very slowly.

Of course, it is important to the DBA that the SGA memory never swap-out to disk. When a memory swap occurs, the swapped task will cease processing for the duration of the swap, until the memory pages are read back-into RAM. This can mean significant slowdowns for Oracle.

The solution for the Oracle DBA is “page fencing”. By marking the SGA as ineligible for swapping, the operating system will ensure that the SGA always stays in RAM memory.

Today, page fencing is available for Solaris and HP/UX. Unfortunately, it is completed undocumented, and the init.ora parameters are buried deep in the release notes. Here are the details.

The above is an excerpt from the "Oracle UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.


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