Hi, On Mon, Apr 27, 2009 at 10:02 PM, Heikki Linnakangas <heikki.linnakan...@enterprisedb.com> wrote: > Fujii Masao wrote: >> >> Hi, >> >> On Mon, Apr 27, 2009 at 8:43 PM, Heikki Linnakangas >> <heikki.linnakan...@enterprisedb.com> wrote: >>> >>> Fujii Masao wrote: >>>> >>>> On Fri, Apr 24, 2009 at 3:20 PM, Heikki Linnakangas >>>> <heikki.linnakan...@enterprisedb.com> wrote: >>>>> >>>>> It's not safe to write WAL after the checkpoint, as RequestXLogSwitch() >>>>> does. After restart, the system will start inserting WAL from the >>>>> checkpoint >>>>> redo point, which is just before the XLOG_SWITCH record, and will >>>>> overwrite >>>>> it. >>>> >>>> Since, in this case, the WAL file including XLOG_SWITCH exists >>>> in archive, I don't think that it's unsafe, i.e. XLOG_SWITCH would >>>> be treated as the last applied record and not be overwritten. WAL >>>> records would start to be inserted from the subsequent file (with >>>> new timeline). >>> >>> It will be overwritten in a normal non-archive-recovery startup. >> >> Hmm, you mean the case where the system crashes after >> inserting XLOG_SWITCH and before archiving the WAL file >> containing it? > > No, no crash is involved. Just a normal server shutdown and start: > > 1. Server shutdown is initiated > 2. A shutdown checkpoint is recorded at XLOG point 1234, redo ptr is also > 1234. > 3. A XLOG_SWITCH record is written at 1235, right after the checkpoint > record. > 4. The last round of archiving is done. The partial WAL file containing the > checkpoint and XLOG_SWITCH record is archived. > 5. Postmaster exits. > > 6. Postmaster is started again. Since the system was shut down cleanly, no > WAL recovery is done. The WAL insert pointer is initialized to right after > the redo pointer, location 1235, which is also the location of the > XLOG_SWITCH record. > 7. The next WAL record written will be written at 1235, overwriting the > XLOG_SWITCH record. > 8. When the WAL file fills up, the system will try to archive the same WAL > file again, this time with additional WAL records that after the checkpoint > record.
Oh, you are right. I've missed that case :( Thanks for the detailed description! Regards, -- Fujii Masao NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
