I have a use case to insert 100 000 rows per min at the same time in another end few threads will take the rows and delete them from my table. So definitely it will create lot of dead tuples in my table.
My auto-vacuum configurations are
autovacuum_max_workers = 3
autovacuum_naptime = 1min
utovacuum_vacuum_scale_factor = 0.2
autovacuum_analyze_scale_factor = 0.1
autovacuum_vacuum_cost_delay = 20ms
autovacuum_vacuum_cost_limit = -1
From "pg_stat_user_tables" I can find auto-vacuum is running on my table but within a few hours my disk will be full (500 GB) and I can't able to insert any new row.
on the second try, I changed the following configuration
autovacuum_naptime = 60min
autovacuum_vacuum_cost_delay = 0
This time my simulation and auto-vacuum are running well and max disk size is 180 GB.
Here my doubt is, if I change the "autovacuum_vacuum_cost_delay" to zero ms, how auto-vacuum freeing the dead tuples space and PG reuse it? why it is not working as intended if I set the value is 20 ms?
Here my doubt is, if I change the "autovacuum_vacuum_cost_delay" to zero ms, how auto-vacuum freeing the dead tuples space and PG reuse it?
The space freed up by vacuum is recorded in the free space map, from where it gets handed out for re-use by future INSERTs.
Another detail to add, in 9.6 the free space map is only vacuumed once the entire table itself is completely vacuumed, and so the freed up space is not findable until then. If the VACUUM never makes it to the very end, because it is too slow or gets interupted, then the space it is freeing up will not be reused for INSERTs. This was improved in v11.
why it is not working as intended if I set the value is 20 ms?
Because vacuum can't keep up at that value. The default values for PostgreSQL are often suitable only for smaller servers, which yours doesn't seem to be. It is appropriate and advisable to change the defaults in this situation. Note that in v12, the default was lowered from 20 to 2 (and its type was correspondingly changed from int to float, so you can now specify the value with more precision)
To summarize, your app creates tons of dead tuples and autovacuum can't keep up. Possible solutions
maintenance_work_mem could help speed autovacuum too. Perhaps you'll find out that you've reached your hard-drive's limits. In that case, you will have to optimize the storage so that it can accommodate those expensive INSERT+DELETE+autovacuum operations.Well the default value is 2 ms Autovacuum. So your 20ms value is high:
autovacuum_vacuum_cost_delay (floating point)
"Specifies the cost delay value that will be used in automatic VACUUM operations. If -1 is specified, the regular vacuum_cost_delay value will be used. If this value is specified without units, it is taken as milliseconds. The default value is 2 milliseconds. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters."
As explained here Vacuum:
" vacuum_cost_delay (floating point)
The amount of time that the process will sleep when the cost limit has been exceeded. If this value is specified without units, it is taken as milliseconds. The default value is zero, which disables the cost-based vacuum delay feature. Positive values enable cost-based vacuuming.
When using cost-based vacuuming, appropriate values for vacuum_cost_delay are usually quite small, perhaps less than 1 millisecond. While vacuum_cost_delay can be set to fractional-millisecond values, such delays may not be measured accurately on older platforms. On such platforms, increasing VACUUM's throttled resource consumption above what you get at 1ms will require changing the other vacuum cost parameters. You should, nonetheless, keep vacuum_cost_delay as small as your platform will consistently measure; large delays are not helpful. "