Hi Albe --- On Wed, 2013/2/6, Albe Laurenz <laurenz.albe@xxxxxxxxxx> wrote: > 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. Many thanks for the answers, its cleared things up for me :) - drew -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
