> On Nov 2, 2018, at 2:39 AM, pavan95 <pavan.postgresdba@xxxxxxxxx> wrote: > > > 1. vacuum_cost_page_hit > 2. vacuum_cost_page_miss > 3. vacuum_cost_page_dirty > > These are tunable parameters to control auto vacuum. Look at vacuum_cost_limit and then consider how the above parameters would affect it. > 1. autovacuum_vacuum_scale_factor=0.2(20%) > 2. autovacuum_analyze_scale_factor=0.1(10%) > Those are just the factors used to determine when auto vacuum or analyze should vacuum/analyze a table. > From the below description: > >> vacuum_cost_page_hit (integer) >> The estimated cost for vacuuming a buffer found in the shared buffer cache. > It represents the cost to lock the buffer pool, lookup the shared hash table > and scan the content of the page. The default value is one > 1. Does the vacuum operate on the buffer pool(memory)? Assuming it is > operated on the data files!!! > 2. What does the sentence "lock the buffer pool, lookup the shared hash > table and scan the content of the page" refers to?? > To do anything with the block in the datafile it first needs to be loaded into the buffer pool thus all associated overhead of that operation. >> vacuum_cost_page_dirty (integer) >> The estimated cost charged when vacuum modifies a block that was previously > clean. It represents the extra I/O required to flush the dirty block out to > disk again. The default value is 20. > What does actually happen here?? Why does this generate extra I/O?? > > What is the meaning of the sentence "It represents the extra I/O required to > flush the dirty block out to disk again"?? Why does vacuum flush the dirty > block to disk when writer process does that? > Again it’s the same as the other tunable parameters.. it just states the cost of modifying a clean block.
