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help / color / mirror / Atom feedFrom: Michael Paquier <[email protected]>
To: Vignesh Raghunathan <[email protected]>
Cc: [email protected]
Subject: Re: Possible mistake in Section 63.6 - 9.6devel Documentation
Date: Tue, 21 Jul 2015 09:59:30 +0900
Message-ID: <CAB7nPqSV1kR6FqWf2rOVter5cXAW8oTH+DKZjwP4Meji5do5wg@mail.gmail.com> (raw)
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On Tue, Jul 21, 2015 at 6:54 AM, Vignesh Raghunathan
<[email protected]> 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
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Attachments:
[text/x-patch] 20150721_pageheader_docfix.patch (2.0K, 2-20150721_pageheader_docfix.patch)
download | inline diff:
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;
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Subject: Re: Possible mistake in Section 63.6 - 9.6devel Documentation
In-Reply-To: <CAB7nPqSV1kR6FqWf2rOVter5cXAW8oTH+DKZjwP4Meji5do5wg@mail.gmail.com>
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