drew_hunt wrote: > I'm trying to get my head around WAL and checkpoints and need to ask a couple > of questions before I > get a headache. > > Firstly, I see the terms "WAL log", "WAL file" and "transaction log" all over > the place - are these > the same thing (i.e. files in the pg_xlog directory)?
Usually they mean the same thing. For exact definitions, read http://www.postgresql.org/docs/current/static/continuous-archiving.html#BACKUP-ARCHIVING-WAL "WAL" consists of a stream of "WAL records" and is physically represented as "WAL segment files" (in pg_xlog). > I'm a bit confused by this paragraph in the docs: > > "Checkpoints are points in the sequence of transactions at which it is > guaranteed that the heap and > index data files have been updated with all information written before the > checkpoint. At checkpoint > time, all dirty data pages are flushed to disk and a special checkpoint > record is written to the log > file. (The changes were previously flushed to the WAL files.)" > > ( see: http://www.postgresql.org/docs/current/static/wal-configuration.html ) > > "a special checkpoint record is written to the log file." > -> which log file is meant here? The WAL. > "The changes were previously flushed to the WAL files." > -> does "previously" here mean "at a previous point in time" or "in > previous PostgreSQL versions"? The former. > -> at what point are changes flushed to WAL files? The change must be on disk in a WAL segment before the transaction can commit. > So say I perform an operation like : > > UPDATE foo SET bar='baz' > > are the following assumptions correct? > > - The first time this changed data hits the disk, it is as an entry in the > WAL log > > - At some point a checkpoint occurs, and the changed data is written to the > actual data file from > system memory (the "dirty data pages"?) > > - the only time the actual data files will be updated from the WAL log (i.e. > not from system memory) > will be after a crash, when the logs are replayed from the last checkpoint? All three are correct. Yours, Laurenz Albe -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
