DEFAULT_CACHE_SIZE mentioned in alert.log of an #Oracle database

Today, I got this message in my alert.log file:

Full DB Caching disabled: DEFAULT_CACHE_SIZE should be at least 709 MBs bigger than current size.

When I look at the datafile sizes and compare them with the buffer cache size, it shows:

 

SYS@cloudcdb > select name,bytes/1024/1024 as mb from v$sgainfo;

NAME                                                       MB
-------------------------------------------------- ----------
Fixed SGA Size                                     2,80265045
Redo Buffers                                       13,1953125
Buffer Cache Size                                        3296
In-Memory Area Size                                      2048
Shared Pool Size                                          736
Large Pool Size                                            32
Java Pool Size                                             16
Streams Pool Size                                           0
Shared IO Pool Size                                       208
Data Transfer Cache Size                                    0
Granule Size                                               16
Maximum SGA Size                                         6144
Startup overhead in Shared Pool                    181,258133
Free SGA Memory Available                                   0

14 rows selected.

SYS@cloudcdb > select sum(bytes)/1024/1024 as mb from v$datafile;

        MB
----------
      3675

It is true, the database doesn’t fit completely into the buffer cache, missing roughly that amount of space mentioned. There is no such parameter as DEFAULT_CACHE_SIZE, though.
What we have instead is DB_CACHE_SIZE. In order to fix that issue, I was using this initialization parameter file to create a new spfile from:

[oracle@uhesse-service2 dbs]$ cat initCLOUDCDB.ora
*.audit_file_dest='/u02/app/oracle/admin/CLOUDCDB/adump'
*.audit_trail='db'
*.compatible='12.1.0.2.0'
*.control_files='/u02/app/oracle/oradata/CLOUDCDB/control01.ctl','/u03/app/oracle/fra/CLOUDCDB/control02.ctl'
*.db_block_size=8192
*.db_domain=''
*.db_name='CLOUDCDB'
*.db_recovery_file_dest='/u03/app/oracle/fra'
*.db_recovery_file_dest_size=10737418240
*.diagnostic_dest='/u02/app/oracle'
*.dispatchers='(PROTOCOL=TCP) (SERVICE=CLOUDCDBXDB)'
*.enable_pluggable_database=true
*.open_cursors=300
*.processes=300
*.remote_login_passwordfile='EXCLUSIVE'
*.undo_tablespace='UNDOTBS1'
*.sga_target=6g
*.pga_aggregate_target=2g
*.inmemory_size=1g
*.db_cache_size=4g

That reduced the size of the In-Memory Column Store to make room for the buffer cache. Now the database fits nicely into the buffer cache again:

SYS@cloudcdb > select name,bytes/1024/1024 as mb from v$sgainfo;

NAME                                                       MB
-------------------------------------------------- ----------
Fixed SGA Size                                     2,80265045
Redo Buffers                                       13,1953125
Buffer Cache Size                                        4256
In-Memory Area Size                                      1024
Shared Pool Size                                          800
Large Pool Size                                            32
Java Pool Size                                             16
Streams Pool Size                                           0
Shared IO Pool Size                                         0
Data Transfer Cache Size                                    0
Granule Size                                               16
Maximum SGA Size                                         6144
Startup overhead in Shared Pool                    181,290176
Free SGA Memory Available                                   0

14 rows selected.

Accordingly the message in the alert.log now reads
Buffer Cache Full DB Caching mode changing from FULL CACHING DISABLED to FULL CACHING ENABLED

Don’t get me wrong: I’m not arguing here against the In-Memory Option or in favor of Full Database Caching. Or whether it makes sense to use any of them or both. This post is just about clarifying the strange message in the alert.log that may confuse people.

And by the way, my demo database is running in the Oracle Cloud 🙂

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