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Document the backup manifest file format.

Browse Source 
Patch by me, at the request of Andres Freund. Reviewed by
Justin Pryzby, Erik Rijkers, Álvaro Herrera, and Andrew
Dunstan.

Discussion: http://postgr.es/m/20200327203225.hcm6ag4grwsiruea@alap3.anarazel.de
pull/51/head
Robert Haas 6 years ago
parent 80634e3b18
commit 149f2ae88a
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  1. 212
      doc/src/sgml/backup-manifest.sgml
  2. 1
      doc/src/sgml/filelist.sgml
  3. 1
      doc/src/sgml/postgres.sgml

212
doc/src/sgml/backup-manifest.sgml
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<!-- doc/src/sgml/backupmanifest.sgml -->
<chapter id="backup-manifest-format">
<title>Backup Manifest Format</title>
<para>
The backup manifest generated by <xref linkend="app-pgbasebackup" /> is
primarily intended to permit the backup to be verified using
<xref linkend="app-pgverifybackup" />. However, it is
also possible for other tools to read the backup manifest file and use
the information contained therein for their own purposes. To that end,
this chapter describes the format of the backup manifest file.
</para>
<para>
A backup manifest is a JSON document encoded as UTF-8. (Although in
general JSON documents are required to be Unicode, PostgreSQL permits
the <type>json</type> and <type>jsonb</type> data types to be used with any
supported server encoding. There is no similar exception for backup
manifests.) The JSON document is always an object; the keys that are present
in this object are described in the next section.
</para>
<sect1 id="backup-manifest-toplevel">
<title>Backup Manifest Toplevel Object</title>
<para>
The backup manifest JSON document contains the following keys.
</para>
<variablelist>
<varlistentry>
<term><literal>PostgreSQL-Backup-Manifest-Version</literal></term>
<listitem>
<para>
The associated value is always the integer 1.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>Files</literal></term>
<listitem>
<para>
The associated value is always a list of objects, each describing one
file that is present in the backup. No entries are present in this
list for the WAL files that are needed in order to use the backup,
or for the backup manifest itself. The structure of each object in the
list is described in <xref linkend="backup-manifest-files" />.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>WAL-Ranges</literal></term>
<listitem>
<para>
The associated value is always a list of objects, each describing a
range of WAL records that must be readable from a particular timeline
in order to make use of the backup. The structure of these objects is
further described in <xref linkend="backup-manifest-wal-ranges" />.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>Manifest-Checksum</literal></term>
<listitem>
<para>
This key is always present on the last line of the backup manifest file.
The associated value is a SHA256 checksum of all the preceding lines.
We use a fixed checksum method here to make it possible for clients
to do incremental parsing of the manifest. While a SHA256 checksum
is significantly more expensive than a CRC32C checksum, the manifest
should normally be small enough that the extra computation won't matter
very much.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="backup-manifest-files">
<title>Backup Manifest File Object</title>
<para>
The object which describes a single file contains either a
<literal>Path</literal> key or an <literal>Encoded-Path</literal> key.
Normally, the <literal>Path</literal> key will be present. The
associated string value is the path of the file relative to the root
of the backup directory. Files located in a user-defined tablespace
will have paths whose first two components are pg_tblspc and the OID
of the tablespace. If the path is not a string that is legal in UTF-8,
or if the user requests that encoded paths be used for all files, then
the <literal>Encoded-Path</literal> key will be present instead. This
stores the same data, but it is encoded as a string of hexadecimal
digits. Each pair of hexadecimal digits in the string represents a
single octet.
</para>
<para>
The following two keys are always present:
</para>
<variablelist>
<varlistentry>
<term><literal>Size</literal></term>
<listitem>
<para>
The expected size of this file, as an integer.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>Last-Modified</literal></term>
<listitem>
<para>
The last modification time of the file as reported by the server at
the time of the backup. Unlike the other fields stored in the backup,
this field is not used by <xref linkend="app-pgverifybackup" />.
It is included only for informational purposes.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
If the backup was taken with file checksums enabled, the following
keys will be present:
</para>
<variablelist>
<varlistentry>
<term><literal>Checksum-Algorithm</literal></term>
<listitem>
<para>
The checksum algorithm used to compute a checksum for this file.
Currently, this will be the same for every file in the backup
manifest, but this may change in future releases. At present, the
supported checksum algorithms are <literal>CRC32C</literal>,
<literal>SHA224</literal>,
<literal>SHA256</literal>,
<literal>SHA384</literal>, and
<literal>SHA512</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>Checksum</literal></term>
<listitem>
<para>
The checksum computed for this file, stored as a series of
hexadecimal characters, two for each byte of the checksum.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="backup-manifest-wal-ranges">
<title>Backup Manifest WAL Range Object</title>
<para>
The object which describes a WAL range always has three keys:
</para>
<variablelist>
<varlistentry>
<term><literal>Timeline</literal></term>
<listitem>
<para>
The timeline for this range of WAL records, as an integer.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>Start-LSN</literal></term>
<listitem>
<para>
The LSN at which replay must begin on the indicated timeline in order to
make use of this backup. The LSN is stored in the format normally used
by <productname>PostgreSQL</productname>; that is, it is a string
consisting of two strings of hexadecimal characters, each with a length
of between 1 and 8, separated by a slash.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>End-LSN</literal></term>
<listitem>
<para>
The earliest LSN at which replay on the indicated timeline may end when
making use of this backup. This is stored in the same format as
<literal>Start-LSN</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Ordinarily, there will be only a single WAL range. However, if a backup is
taken from a standby which switches timelines during the backup due to an
upstream promotion, it is possible for multiple ranges to be present, each
with a different timeline. There will never be multiple WAL ranges present
for the same timeline.
</para>
</sect1>
</chapter>

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doc/src/sgml/filelist.sgml
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<!ENTITY storage SYSTEM "storage.sgml">
<!ENTITY tablesample-method SYSTEM "tablesample-method.sgml">
<!ENTITY generic-wal SYSTEM "generic-wal.sgml">
<!ENTITY backup-manifest SYSTEM "backup-manifest.sgml">
<!-- contrib information -->
<!ENTITY contrib SYSTEM "contrib.sgml">

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doc/src/sgml/postgres.sgml
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@ -268,6 +268,7 @@ break is not needed in a wider output rendering.
&storage;
&bki;
&planstats;
&backup-manifest;
</part>

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