On Sat, Apr 20, 2019 at 12:09:42AM -0400, Robert Haas wrote: > On Thu, Apr 18, 2019 at 5:47 PM Bruce Momjian <br...@momjian.us> wrote: > > How would the modblock file record all the modified blocks across > > restarts and crashes? I assume that 1G of WAL would not be available > > for scanning. I suppose that writing a modblock file to some PGDATA > > location when WAL is removed would work since during a crash the > > modblock file could be updated with the contents of the existing pg_wal > > files. > > I think you've got to prevent the WAL from being removed until a > .modblock file has been written. In more detail, you should (a) scan > all the WAL segments that will be summarized in the .modblock file, > (b) write the file under a temporary name, (c) fsync it, (d) rename it > into place, (e) fsync it again, and (f) then allow those WAL segments > to be removed, if they are otherwise eligible to be removed.
Makes sense. So when you are about to remove WAL, you create the .modblock files for all complete WAL files and only create a new one when you are about to remove a WAL that was not in a previous .modblock file. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + As you are, so once was I. As I am, so you will be. + + Ancient Roman grave inscription +
