{"id":3869,"date":"2021-07-15T16:23:05","date_gmt":"2021-07-15T16:23:05","guid":{"rendered":"https:\/\/nenadnoveljic.com\/blog\/?p=3869"},"modified":"2021-07-15T16:23:08","modified_gmt":"2021-07-15T16:23:08","slug":"minimum-sga_target","status":"publish","type":"post","link":"https:\/\/nenadnoveljic.com\/blog\/minimum-sga_target\/","title":{"rendered":"Minimum sga_target"},"content":{"rendered":"

We occasionally got the following error message when attempting to start the databases after a hardware change or during a database upgrade:<\/p>\n

ORA-00821: Specified value of sga_target 4096M is too small, needs to be at least 4352M<\/code><\/pre>\n
We immediately resolved the issue by increasing sga_target, but couldn’t explain the root cause.<\/p>\n
Obviously, the hardware configuration has an impact on the minimum sga_target calculation, and the limit usually changes with a new database release.<\/p>\n
Unfortunately, this calculation isn’t documented. And yet it would be extremely useful to anticipate the new limit and avoid unpleasant suprises.<\/p>\n
I asked for information on Oracle-l<\/a>.<\/p>\n
Jonathan Lewis<\/a> said that the calculation depends on the database parameter cpu_count<\/a>. Sayan Malakshinov<\/a> mentioned the processes parameter<\/a>.<\/p>\n
I setup an experiment to measure how the minimum sga_sarget depends on both values.<\/p>\n
Experiment<\/h1>\n
After backing up the spfile, I was repeating the following process with different parameter values:<\/p>\n
    \n
  1. set sga_target to 100M<\/li>\n
  2. adjust the parameter of interest<\/li>\n
  3. stop the database<\/li>\n
  4. try to start the database (it will fail.)<\/li>\n
  5. get the new limit from the error message<\/li>\n
  6. restore the correct spfile<\/li>\n<\/ol>\n
    I semi-automated the testing with the code snippet below:<\/p>\n
    Backup spfile (only once):<\/p>\n
    cp spfileDB.ora spfileDB.ora.bak<\/code><\/pre>\n
    Change cpu_count (or any other parameter) with every execution:<\/p>\n
    alter system set cpu_count=192 scope=spfile ;\nalter system set sga_target=100M scope=spfile ;\nshutdown abort\nstartup\n\n! cp spfileDB.ora.bak spfileDB.ora\nshutdown abort\nstartup<\/code><\/pre>\n
    Model<\/h1>\n
    After reducing cpu_count from 120 to 60, the minimum sga_target approximately halved.<\/p>\n
    In contrast, the parameter processes had a neglectable impact on the requirement.<\/p>\n
    Since the dependency on cpu_count is linear, and cpu_count is the main contributor, we can approximate all other dependencies with a constant C:<\/p>\n
    min_sga_target = k * cpu_count + C<\/p>\n
    After completing experiments with two different cpu_count values we can find k (slope) and C (y intercept) by solving the system of two linear equations, for example:<\/p>\n
    cpu_count = 120, min_sga_target =  2576M
    \ncpu_count = 60, min_sga_target = 1348M<\/p>\n
    k = 20.47M , C = 120M<\/p>\n
    We can verify this calculation by picking a third value, calculating it and comparing with reality:<\/p>\n
    cpu_count = 30, min_sga_target = 734M<\/p>\n
    ORA-00821: Specified value of sga_target 100M is too small, needs to be at least 764M<\/code><\/pre>\n
    As we can see, the calculation is reliable. We can use it to predict the new minimum sga_target after adding more CPUs.<\/p>\n
    Bounds<\/h1>\n
    The extrapolation works for the following cpu_count values:<\/p>\n
    1 << cpu_count < #cpu<\/p>\n
    In other words, cpu_count has to be siginficantly greater than 1 and lower or equal than the number of CPU threads on the server. Simply put, we’re looking at large servers.<\/p>\n
    Lower bound<\/h2>\n
    Fow a low cpu_count the dependency becomes non-linear, as there are some minimal requirements for sga_target, for example for cpu_count=4<\/p>\n
    ORA-00821: Specified value of sga_target 100M is too small, needs to be at least 296M<\/code><\/pre>\n
    But that’s irrelevant, because the theoretical value for minimum sga_target for a low cpu_count is so small that it’s unfit for the real world.<\/p>\n
    Upper bound<\/h2>\n
    Minimum sga_target doesn’t grow anymore after increasing cpu_count above the total number of CPUs.<\/p>\n
    For example, for cpu_count=192<\/p>\n
    ORA-00821: Specified value of sga_target 100M is too small, needs to be at least 2576M<\/code><\/pre>\n
    Risks<\/h1>\n
    Hardware and OS changes<\/h2>\n
    Adding more CPUs to the server increases the minimum sga_target requirement, which can prevent the restart of the databases that suddenly come below the new limit.<\/p>\n
    Notice that the database sees hyperthreads as CPUs, so adding virtual CPUs or turning on hyperthreading can also lead to the problem.<\/p>\n
    Or if your test server has less CPUs, some problems might only appear in production.<\/p>\n
    Oracle upgrades<\/h2>\n
    Here are slope and y intercept values for 19.10.0.0.210119 in MB: k = 20.46 , C = 120<\/p>\n
    Let’s compare them with 12.2.0.1.180116: k = 8.47, C = 124<\/p>\n
    The minimum sga_target is 2.4 time higher in 19c than in 12c.<\/p>\n
    Monitoring<\/h1>\n
    After becoming aware of the risk, we started monitoring sga_target with the following Perl code:<\/p>\n
      sub get_min_sga_target {\n    my ($self) = shift @_ ;\n    \n    my $cpu_count = $self->get_spfile_parameter('cpu_count') ;\n    \n    return $cpu_count * 21 * 1024 ** 2 + 1024 ** 3 ;\n  }<\/code><\/pre>\n
    use Number::Format qw( format_bytes ) ;\nsub exec_check{\n  my $db_obj = shift @_ ;\n\n  my $sga_target = $db_obj->get_spfile_parameter('sga_target') ;  \n  \n  my $min_sga_target = $db_obj->get_min_sga_target() ; \n\n  my $return_code = ( $sga_target < $min_sga_target ) ? 1 : 0 ;\n  \n  $sga_target = format_bytes( $sga_target, unit => 'G' ) ;\n  $min_sga_target = format_bytes( $min_sga_target, unit => 'G' ) ;\n  \n  my $message \n    = \"sga_target: $sga_target, minimum: $min_sga_target\" ;\n\n  return { return_code => $return_code , message => $message } ;  \n}<\/code><\/pre>\n
    Conclusion<\/h1>\n
    In conclusion, minimum sga_target largely depends on cpu_count. Consequently, database start might fail after adding CPUs. This includes virtual CPUs and hyperthreads as well.<\/p>\n
    The limit changes with every database release, so an upgrade can fail for databases with a smaller SGA. Also, a database upgrade that you tested on a server with much less CPUs might fail in production.<\/p>\n
    I provided the methodology to anticipate problems and monitor critical conditions.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Database won’t start after a hardware\/OS change or a database upgrade?<\/p>\n
    You can monitor critical conditions and prevent unpleasant surprises. Continue Reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[5],"tags":[],"class_list":["post-3869","post","type-post","status-publish","format-standard","hentry","category-oracle"],"yoast_head":"\nMinimum sga_target - All-round Database Topics<\/title>\n<meta name=\"description\" content=\"Database won't start after a hardware\/OS change or a database upgrade?You can monitor critical conditions and prevent unpleasant surprises.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/nenadnoveljic.com\/blog\/minimum-sga_target\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Minimum sga_target - 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