On Tue, Jul 21, 2015 at 6:54 AM, Vignesh Raghunathan <vignesh.pg...@gmail.com> wrote: > Hello, > > It has been mentioned in Section 63.6 that the first two fields in > PageHeaderData track the most recent WAL entry related to the page. However, > I am not sure how pd_checksum is related to WAL. Could it be possible that > the sentence has been carried over from previous versions of the > documentations without considering the change to the second field in > PageHeaderData?
Yes, the documentation is mistaken. The two bytes of pd_tli have been switched to pd_checksum in 9.3, hence only the first field is relevant for WAL, aka pd_lsn. Looking at this portion of the docs I think that it should be updated as attached, mentioning pd_checksum as well. -- Michael
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml index e5b7b4b..073fe2b 100644 --- a/doc/src/sgml/storage.sgml +++ b/doc/src/sgml/storage.sgml @@ -758,18 +758,18 @@ data. Empty in ordinary tables.</entry> <para> The first 24 bytes of each page consists of a page header - (PageHeaderData). Its format is detailed in <xref - linkend="pageheaderdata-table">. The first two fields track the most - recent WAL entry related to this page. Next is a 2-byte field - containing flag bits. This is followed by three 2-byte integer fields - (<structfield>pd_lower</structfield>, <structfield>pd_upper</structfield>, - and <structfield>pd_special</structfield>). These contain byte offsets - from the page start to the start - of unallocated space, to the end of unallocated space, and to the start of - the special space. - The next 2 bytes of the page header, - <structfield>pd_pagesize_version</structfield>, store both the page size - and a version indicator. Beginning with + (<structname>PageHeaderData</>). Its format is detailed in <xref + linkend="pageheaderdata-table">. The first field tracks the most + recent WAL entry related to this page. The second field contains + the page checksum if <xref linkend="app-initdb-data-checksums"> are + enabled. Next is a 2-byte field containing flag bits. This is followed + by three 2-byte integer fields (<structfield>pd_lower</structfield>, + <structfield>pd_upper</structfield>, and + <structfield>pd_special</structfield>). These contain byte offsets + from the page start to the start of unallocated space, to the end of + unallocated space, and to the start of the special space. The next 2 + bytes of the page header, <structfield>pd_pagesize_version</structfield>, + store both the page size and a version indicator. Beginning with <productname>PostgreSQL</productname> 8.3 the version number is 4; <productname>PostgreSQL</productname> 8.1 and 8.2 used version number 3; <productname>PostgreSQL</productname> 8.0 used version number 2;
-- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs
