Block Change Tracking Not Used Though Enabled
During normal checks of the backups, we observed the
backups are taking longer than usual. These are QA databases which are not
backed up thru schedules. We use Ad hoc backups for these hosts to conserve
space. Also we usually take monthly full and rest of the days L1 incremental
backups. Also the change i.e. block
change tracking was implemented recently on these hosts. We double checked that
earlier runs were finishing with BCT in around couple of hours which has now
gone back to previous run times of 7-8 hours. Strange that why this happened??!!
Following confirms what we were observing.
BCT is enabled and alive.
Then we checked whether datafile backup is using BCT or
not.
select completion_time, datafile_blocks, blocks_read,
blocks, used_change_tracking
from v$backup_datafile where file#=8 order by 1;
We also checked the OS process, responsible for writing
changed blocks to BCT file and found it running on both nodes of cluster .
[oracle@ rman]$ ps
-ef | grep ctwr
oracle 6284 1 0
Jul26 ? 00:00:00 ora_ctwr_xxxxx1
Alert log also shows that process is active as well–
CTWR started with pid=65, OS id=2653
Block change tracking service is active.
So, we have everything looking healthy but why the BCT is
still not used. We researched a bit on MOS and we got the reason why it was not
used.
Cause –
If L1 backups are more than 8
then you need to increase the parameter _bct_bitmaps_per_file (init.or or
spfile setting) from its default of 8 to the number of incremental that one take
between full backups. Also, if you have taken more than 8
incremental backups since the last full back up, you need to take another full back
up before the change will kick in, because the historical bitmaps have already
been deleted.
When I checked the number of L1 taken since last full
backup they were in excess of 8!! So we
decided to take full backup and after that when we checked incremental, the
issue was resolved.
Lesson learned –
If there are 8 or more than 8 L1 backups since last L0,
the BCT will not be used for backups.
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