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Wednesday, 22 June 2011

Paging vs Swapping

These two terms are associated with the virtual filesystem.
In the past, the cost of RAM (physical memory) is relatively high compared to the cost of hard disk drives.
During then, it was almost always preferrable to use the virtual file system.
In a nut shell, virtual filesystem is combining the use of  physical RAM and a particular slice or file in the hard disk  as the total memory.
In the event the system runs out of physical memory and a more important job is waiting to be executed it transfer some of its load in the RAM to the swap memory.
The term paging and swapping refers to the transfer of data from RAM to the HDD. They only differ on the bulk of data they transfer. In paging only portion on the data in the memory is being transferred while in swapping ALL data is being transfered.
Paging happens in normal operation. In many cases, it is totally inevitable not to page some of the memory contents to the swap space. Especially, on development servers where in the coreadm is using the swap space for its corefiles.
On the other hand, swapping happens when there is an abnormal event in the system. This happens very rarely, and if it does there must be something wrong with the configuration of the server.
Its very unlikely for a unix box to experience swapping.

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