What influences the time taken for the 'alter tablespace begin backup'
statement?
Until a couple of weeks ago, I could get my entire database (50
tablespaces, 1TB) into backup mode in about 10-15 minutes, but then
all of a sudden it started taking 45-50 minutes to do the same action.
The only change I am aware of which coincided, was an increase by 4GB
(from 12GB to 16GB) in the data cache (db_block_buffers). I am
interested to understand what actually happens as the 'begin backup'
statement is executed. What does it wait for?