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Don Burleson Blog 








Don?t waste RAM on 32-bit Oracle Windows servers

Oracle Tips by Burleson Consulting

I grew-up believing that wasting something was a mortal sin against God, and I remain very stingy with server resources as an Oracle DBA.  One of the largest wastes I see are Oracle Windows servers that some with 4-gig or 8-gig RAM, but the SGA is only sized to 1.5 gig.  This leaves many gigabytes of valuable RAM memory going to waste!

Wasting RAM and this is very common on 32-bit Oracle Windows databases, especially servers with 8 gig RAM and a low high-water mark for external PGA RAM usage.  As a refresher, when you don't have enough RAM to fully-cache your working-set of frequently-referenced table and index blocks, your database does unnecessary I/O:




The Solutions to Windows RAM waste:

There are two solutions used by most shops.  You have two choices, 4GT or AWE.  To enable 4GT, add the /3GB switch to the Boot.ini file. AWE is best and you can use the entire RAM for the Oracle database (less 20% for the OS).

In my experience the increased data caching can make a huge difference and for a dedicated Windows server you only need to reserve 20% of the RAM for the OS, and save the rest for SGA and PGA.



- In Windows NT Server v4.0 EE, 4 gig RAM Tuning (4GT) was added which allows SGA up to 3 gig of memory as opposed to the standard 2 gig. (Yes, 4GT = 3 gig)

- Gives Oracle up to 50% more memory. (but still not enough)

- For larger systems 64-bit Oracle allows up to 512 gig SGA sizes.

- All that's needed is an added boot.ini switch -- /3GB



- AWE (Address Windowing Extension) - Allows 32-bit Windows Oracle to have up to 64 gig of database buffers.

- Available after Oracle8i (8.1.6).

- Moves database buffers out of the 3GB address space "above the line", leaving low-memory RAM for more connections, larger sort areas, etc...

- The Win32 AWE (Address Windowing Extension) calls are used which provide a fast map/unmap interface to "above the line" RAM.

- When running on a machine with >16GB of RAM, Oracle can either use the 4GT feature or the memory above 16GB, but not both.  This is a Windows limitation.

- From the Oracle 3 gig address space, we allocate space for a 'window' onto the whole of the db buffers. 

- The size of this window is user-configurable. 

- The larger the window, the more buffers are quickly available, however the larger the window, the less space there is for other memory structures like shared pool, connections, PGAs, etc.


- Set the parameter use_indirect_data_buffers=true

- Set AWE_WINDOW_SIZE in the registry to the number of bytes of address space to use for your window onto extended memory.  The default is 1GB.

- Increase db_block_buffers to a higher value (expressed in blocks) up to available memory.

For more information on sizing RAM regions for minimum wastage, see my book "Oracle Tuning: The Definitive Reference". 

If you like Oracle tuning, see the 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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