Re: pg_wal lifecycle

On 13.07.21 09:07, Luca Ferrari wrote:
> I'd like to see if I get it right about pg_wal: it grows up to pretty
> much max_wal_size, at which point a checkpoint is triggered.
> If the server triggers a timed checkpoint before the pg_wal is at
> max_wal_size, the system recycles the wals thus keeping the pg_wal
> size lower than max_wal_size.
> Is this correct? In particular, what is in simple words, the
> discrimination between recycling a segment and creating a new one?

Recycling in this context just means that instead of creating a new WAL 
file for new WAL traffic, it reuses an old file.  So if you have WAL 
files 5, 6, 7, 8, 9, and you know that you don't need 5 and 6 anymore, 
when you need to start WAL file 10, instead of creating a new file "10", 
the system just renames "5" to "10" and starts overwriting what was in 
there.  This is just an optimization to use the file system better; it 
doesn't affect the logical principles of what is going on.