{"id":10,"date":"2015-02-09T14:31:47","date_gmt":"2015-02-09T14:31:47","guid":{"rendered":"https:\/\/dbadave.co.uk\/?p=10"},"modified":"2016-02-10T11:32:38","modified_gmt":"2016-02-10T11:32:38","slug":"excessive-deletes-from-smon_scn_time","status":"publish","type":"post","link":"https:\/\/dbadave.co.uk\/excessive-deletes-from-smon_scn_time\/","title":{"rendered":"Excessive Deletes From SMON_SCN_TIME"},"content":{"rendered":"

It’s Always Friday Afternoon!<\/h2>\n

I got a support call late last Friday afternoon (why do most calls seem to come on a Friday afternoon just before you finish for the weekend?) about bad performance on one of our applications we host for a customer.<\/p>\n

When I logged into the server and took a look at what it was doing, the CPU was getting hammered by the Oracle SMON process.<\/p>\n

The Problem – Excessive Deletes From SMON_SCN_TIME<\/h2>\n

As it was just past the hour, I generated an AWR report for the previous hour and I could immediately see the problem – the following SQL statement had been executed over 40,000 times and was responsible for the vast majority of DB Time:<\/p>\n

delete from smon_scn_time where thread=0 and scn = (select min(scn) from smon_scn_time where thread=0)<\/em><\/p>\n

A quick check on Metalink<\/a> (Oracle’s support site) turned up document ID\u00a0375401.1 High Executions Of Statement “delete from smon_scn_time…”<\/p>\n

According to this document the problem affects database versions from 10.1.0.2 to 10.2.0.4 and is described as follows:<\/p>\n

The delete statement deletes the oldest rows from smon_scn_time to clear space for new rows.\u00a0 SMON wakes up every 5 minutes and checks how many on-disk mappings we have–the max is 144000.<\/em><\/p>\n

The new mappings are then added for the last period (since SMON last updated), and if this is over 144000, SMON will then issue the delete statement:<\/em><\/p>\n

delete from smon_scn_time where thread=0 and time_mp = (select min(time_mp) from smon_scn_time where thread=0)<\/em><\/p>\n

There will be an execution of this each time SMON wakes to update smon_scn_time, and if one deletion does not free enough mappings, then there will be multiple executions.<\/em><\/p>\n

What happens is due to the inconsistency between the table and indexes the delete returns zero rows; so the delete statement is executed continuously to reduce the smon_scn_time below the maximum 14400 mappings.<\/em><\/p>\n

Validating the Problem<\/h2>\n

You can check the inconsistency by validating the structure from within SQL*Plus:<\/p>\n

SQL> analyze table smon_scn_time validate structure cascade;<\/em><\/p>\n

When I did this check, I got the following error:<\/p>\n

SQL> analyze table smon_scn_time validate structure cascade;<\/em><\/p>\n

analyze table smon_scn_time validate structure cascade<\/em>
\n*<\/em>
\nERROR at line 1 :<\/em>
\nORA-01499: table\/Index Cross Reference Failure – see trace file<\/em><\/p>\n

The Solution<\/h2>\n

It did indeed look as though this was the cause of my issue so I followed the steps recommended by Oracle Support to fix it:<\/p>\n

    \n
  1. Make a backup in case of problems<\/li>\n
  2. Drop index smon_scn_time_scn_idx;<\/li>\n
  3. Drop index smon_scn_time_tim_idx;<\/li>\n
  4. Create unique index smon_scn_time_scn_idx on smon_scn_time(scn);<\/li>\n
  5. Create unique index smon_scn_time_tim_idx on smon_scn_time(time_mp);<\/li>\n<\/ol>\n

    After doing this, performance returned to previous normal levels and just to check, I analyzed the table smon_scn_time again – this time no errors were returned.<\/p>\n

    So if you suddenly find you’re suffering performance problems due to excessive deletes from smon_scn_time, follow the steps above to fix it.<\/p>\n","protected":false},"excerpt":{"rendered":"

    I got a support call late last Friday afternoon (why do most call seem to come on a Friday afternoon just before you finish for the weekend?) about bad performance on one of our applications we host for a customer.<\/p>\n

    When I logged into the server and took a look at what it was doing, the CPU was getting hammered by the Oracle SMON process.<\/p>\n","protected":false},"author":1,"featured_media":388,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13],"tags":[4,5,12],"yoast_head":"\nExcessive Deletes From SMON_SCN_TIME | DBADave<\/title>\n<meta name=\"description\" content=\"The other day I had an Oracle database performance issue that turned out to be due to excessive deletes from SMON_SCN_TIME. 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