Strange Behavior with MEMORY_TARGET

I got a help request from a client who was running Oracle 11.1.0.6 64-bit. Their memory_target parameter was set to 5G, with sga_target and all pool parameters set to 0. pga_aggregate_target was explicitly set to 750MB. However, Oracle would not allocate more than 64MB RAM to the Buffer Cache. We looked through multiple snapshots and never found a case where db_cache_size was above 64MB, despite a staggering amount of disk reads. So we tried to change the db_cache_size manually to set the minimum:

SQL> alter system set db_cache_size = 1024M;
alter system set db_cache_size = 1024M
*
ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache

I wanted to see if I could duplicate the issue, so I tested it on my 11.1.0.6 Windows 64-bit instance. I prepared the system by setting pga_aggregate_target to 128M, sga_target to 512M, and memory_target = 1648M.

NOTE: Throughout these examples, I’ve removed the irrelevant results from “show parameter target”.

Setting It Up

SQL> alter system set pga_aggregate_target = 128M;

System altered.

SQL> alter system set sga_target = 512M;

System altered.

SQL> alter system set memory_target = 1648M;

System altered.

SQL> show parameter target

NAME                             TYPE        VALUE
-------------------------------- ----------- -----
memory_max_target                big integer 1648M
memory_target                    big integer 1648M
pga_aggregate_target             big integer 128M
sga_target                       big integer 512M

SQL> show sga

Total System Global Area 1720328192 bytes
Fixed Size                  2115656 bytes
Variable Size            1426065336 bytes
Database Buffers          285212672 bytes
Redo Buffers                6934528 bytes

The next step was to test the change:

Testing the Change

SQL> alter system set db_cache_size = 1024M;
alter system set db_cache_size = 1024M
*
ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache

So I tried turning off memory_target, and look what happens to the PGA:

Turn off memory_target

SQL> alter system set memory_target = 0;

System altered.

SQL> show parameter target

NAME                             TYPE        VALUE
-------------------------------- ----------- -----
memory_max_target                big integer 1648M
memory_target                    big integer 0
pga_aggregate_target             big integer 1136M
sga_target                       big integer 512M

The PGA was given all of the extra space! This was pretty strange, so I went ahead and changed the PGA to 128M and tried my test again:

Try again

SQL> alter system set pga_aggregate_target = 128M;

System altered.

SQL> show parameter target

NAME                             TYPE        VALUE
-------------------------------- ----------- -----
memory_max_target                big integer 1648M
memory_target                    big integer 0
pga_aggregate_target             big integer 128M
sga_target                       big integer 512M

SQL> alter system set db_cache_size = 1024M;
alter system set db_cache_size = 1024M
*
ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache

Even then, it would not let me grow the buffer cache. I had to disable sga_target for 11g to finally allow it:

Solution

SQL> alter system set sga_target = 0;

System altered.

SQL> alter system set db_cache_size = 1024M;

System altered.

SQL> alter system set memory_target = 1648M;

System altered.

SQL> show parameter target

NAME                             TYPE        VALUE
-------------------------------- ----------- -----
memory_max_target                big integer 1648M
memory_target                    big integer 1648M
pga_aggregate_target             big integer 128M
sga_target                       big integer 0

Is this expected behavior from memory_target? Did I miss something? Is it a bug in 11.1.0.6? I was not able to find any notes on Metalink regarding the issue. The Oracle Documentation states that this should work just fine. When memory_target is enabled, sga_target and pga_aggregate_target should only work as minimums if explicitly set.

I will play around with it some more if I get the chance. In the meantime, I have to wonder how many DBAs have confidently set memory_target, all the while not knowing their individual pools weren’t being sized properly?

Update 1

I just did another test where I “primed the pump” so to speak. To do so, I set the sga_target higher. It allowed me to grow the db_cache_size at that point. However, I then set db_cache_size down to 0, set sga_target back to 512M, and was still able to set db_cache_size back up to 1024M afterwards.

NAME                         TYPE        VALUE
---------------------------- ----------- -----
memory_max_target            big integer 1648M
memory_target                big integer 1648M
pga_aggregate_target         big integer 128M
sga_target                   big integer 512M

SQL> alter system set db_cache_size = 1024M;
alter system set db_cache_size = 1024M
*
ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache

SQL> alter system set sga_target = 1300M;

System altered.

SQL> alter system set db_cache_size = 1024M;

System altered.

SQL> alter system set db_cache_size = 0;

System altered.

SQL> alter system set sga_target = 512M;

System altered.

SQL> show parameter target

NAME                         TYPE        VALUE
---------------------------- ----------- -----
memory_max_target            big integer 1648M
memory_target                big integer 1648M
pga_aggregate_target         big integer 128M
sga_target                   big integer 512M

SQL> alter system set db_cache_size = 1024M;

System altered.

SQL> show parameter target

NAME                         TYPE        VALUE
---------------------------- ----------- -----
memory_max_target            big integer 1648M
memory_target                big integer 1648M
pga_aggregate_target         big integer 128M
sga_target                   big integer 512M

SQL> show parameter db_cache_size

NAME                             TYPE        VALUE
-------------------------------- ----------- -----
db_cache_size                    big integer 1G

The end looks just like the beginning. The only difference would be an internal barrier being lifted.