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Use the terminology "WAL file" not "log file" more consistently.

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Referring to the WAL as just "log" invites confusion with the
postmaster log, so avoid doing that in docs and error messages.
Also shorten "WAL segment file" to just "WAL file" in various
places.

Bharath Rupireddy, reviewed by Nathan Bossart and Kyotaro Horiguchi

Discussion: https://postgr.es/m/CALj2ACUeXa8tDPaiTLexBDMZ7hgvaN+RTb957-cn5qwv9zf-MQ@mail.gmail.com
This commit is contained in:
Tom Lane
2022-09-14 18:40:58 -04:00
parent 63840526b0
commit 31dcfae83c
14 changed files with 67 additions and 67 deletions
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60
doc/src/sgml/wal.sgml
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@@ -297,12 +297,12 @@
transaction processing. Briefly, <acronym>WAL</acronym>'s central
concept is that changes to data files (where tables and indexes
reside) must be written only after those changes have been logged,
that is, after log records describing the changes have been flushed
that is, after WAL records describing the changes have been flushed
to permanent storage. If we follow this procedure, we do not need
to flush data pages to disk on every transaction commit, because we
know that in the event of a crash we will be able to recover the
database using the log: any changes that have not been applied to
the data pages can be redone from the log records. (This is
the data pages can be redone from the WAL records. (This is
roll-forward recovery, also known as REDO.)
</para>
@@ -323,15 +323,15 @@
<para>
Using <acronym>WAL</acronym> results in a
significantly reduced number of disk writes, because only the log
significantly reduced number of disk writes, because only the WAL
file needs to be flushed to disk to guarantee that a transaction is
committed, rather than every data file changed by the transaction.
The log file is written sequentially,
and so the cost of syncing the log is much less than the cost of
The WAL file is written sequentially,
and so the cost of syncing the WAL is much less than the cost of
flushing the data pages. This is especially true for servers
handling many small transactions touching different parts of the data
store. Furthermore, when the server is processing many small concurrent
transactions, one <function>fsync</function> of the log file may
transactions, one <function>fsync</function> of the WAL file may
suffice to commit many transactions.
</para>
@@ -341,10 +341,10 @@
linkend="continuous-archiving"/>. By archiving the WAL data we can support
reverting to any time instant covered by the available WAL data:
we simply install a prior physical backup of the database, and
replay the WAL log just as far as the desired time. What's more,
replay the WAL just as far as the desired time. What's more,
the physical backup doesn't have to be an instantaneous snapshot
of the database state &mdash; if it is made over some period of time,
then replaying the WAL log for that period will fix any internal
then replaying the WAL for that period will fix any internal
inconsistencies.
</para>
</sect1>
@@ -497,15 +497,15 @@
that the heap and index data files have been updated with all
information written before that checkpoint. At checkpoint time, all
dirty data pages are flushed to disk and a special checkpoint record is
written to the log file. (The change records were previously flushed
written to the WAL file. (The change records were previously flushed
to the <acronym>WAL</acronym> files.)
In the event of a crash, the crash recovery procedure looks at the latest
checkpoint record to determine the point in the log (known as the redo
checkpoint record to determine the point in the WAL (known as the redo
record) from which it should start the REDO operation. Any changes made to
data files before that point are guaranteed to be already on disk.
Hence, after a checkpoint, log segments preceding the one containing
Hence, after a checkpoint, WAL segments preceding the one containing
the redo record are no longer needed and can be recycled or removed. (When
<acronym>WAL</acronym> archiving is being done, the log segments must be
<acronym>WAL</acronym> archiving is being done, the WAL segments must be
archived before being recycled or removed.)
</para>
@@ -544,7 +544,7 @@
another factor to consider. To ensure data page consistency,
the first modification of a data page after each checkpoint results in
logging the entire page content. In that case,
a smaller checkpoint interval increases the volume of output to the WAL log,
a smaller checkpoint interval increases the volume of output to the WAL,
partially negating the goal of using a smaller interval,
and in any case causing more disk I/O.
</para>
@@ -614,10 +614,10 @@
<para>
The number of WAL segment files in <filename>pg_wal</filename> directory depends on
<varname>min_wal_size</varname>, <varname>max_wal_size</varname> and
the amount of WAL generated in previous checkpoint cycles. When old log
the amount of WAL generated in previous checkpoint cycles. When old WAL
segment files are no longer needed, they are removed or recycled (that is,
renamed to become future segments in the numbered sequence). If, due to a
short-term peak of log output rate, <varname>max_wal_size</varname> is
short-term peak of WAL output rate, <varname>max_wal_size</varname> is
exceeded, the unneeded segment files will be removed until the system
gets back under this limit. Below that limit, the system recycles enough
WAL files to cover the estimated need until the next checkpoint, and
@@ -650,7 +650,7 @@
which are similar to checkpoints in normal operation: the server forces
all its state to disk, updates the <filename>pg_control</filename> file to
indicate that the already-processed WAL data need not be scanned again,
and then recycles any old log segment files in the <filename>pg_wal</filename>
and then recycles any old WAL segment files in the <filename>pg_wal</filename>
directory.
Restartpoints can't be performed more frequently than checkpoints on the
primary because restartpoints can only be performed at checkpoint records.
@@ -676,12 +676,12 @@
insertion) at a time when an exclusive lock is held on affected
data pages, so the operation needs to be as fast as possible. What
is worse, writing <acronym>WAL</acronym> buffers might also force the
creation of a new log segment, which takes even more
creation of a new WAL segment, which takes even more
time. Normally, <acronym>WAL</acronym> buffers should be written
and flushed by an <function>XLogFlush</function> request, which is
made, for the most part, at transaction commit time to ensure that
transaction records are flushed to permanent storage. On systems
with high log output, <function>XLogFlush</function> requests might
with high WAL output, <function>XLogFlush</function> requests might
not occur often enough to prevent <function>XLogInsertRecord</function>
from having to do writes. On such systems
one should increase the number of <acronym>WAL</acronym> buffers by
@@ -724,7 +724,7 @@
<varname>commit_delay</varname>, so this value is recommended as the
starting point to use when optimizing for a particular workload. While
tuning <varname>commit_delay</varname> is particularly useful when the
WAL log is stored on high-latency rotating disks, benefits can be
WAL is stored on high-latency rotating disks, benefits can be
significant even on storage media with very fast sync times, such as
solid-state drives or RAID arrays with a battery-backed write cache;
but this should definitely be tested against a representative workload.
@@ -828,16 +828,16 @@
<para>
<acronym>WAL</acronym> is automatically enabled; no action is
required from the administrator except ensuring that the
disk-space requirements for the <acronym>WAL</acronym> logs are met,
disk-space requirements for the <acronym>WAL</acronym> files are met,
and that any necessary tuning is done (see <xref
linkend="wal-configuration"/>).
</para>
<para>
<acronym>WAL</acronym> records are appended to the <acronym>WAL</acronym>
logs as each new record is written. The insert position is described by
files as each new record is written. The insert position is described by
a Log Sequence Number (<acronym>LSN</acronym>) that is a byte offset into
the logs, increasing monotonically with each new record.
the WAL, increasing monotonically with each new record.
<acronym>LSN</acronym> values are returned as the datatype
<link linkend="datatype-pg-lsn"><type>pg_lsn</type></link>. Values can be
compared to calculate the volume of <acronym>WAL</acronym> data that
@@ -846,12 +846,12 @@
</para>
<para>
<acronym>WAL</acronym> logs are stored in the directory
<acronym>WAL</acronym> files are stored in the directory
<filename>pg_wal</filename> under the data directory, as a set of
segment files, normally each 16 MB in size (but the size can be changed
by altering the <option>--wal-segsize</option> <application>initdb</application> option). Each segment is
divided into pages, normally 8 kB each (this size can be changed via the
<option>--with-wal-blocksize</option> configure option). The log record headers
<option>--with-wal-blocksize</option> configure option). The WAL record headers
are described in <filename>access/xlogrecord.h</filename>; the record
content is dependent on the type of event that is being logged. Segment
files are given ever-increasing numbers as names, starting at
@@ -861,7 +861,7 @@
</para>
<para>
It is advantageous if the log is located on a different disk from the
It is advantageous if the WAL is located on a different disk from the
main database files. This can be achieved by moving the
<filename>pg_wal</filename> directory to another location (while the server
is shut down, of course) and creating a symbolic link from the
@@ -877,19 +877,19 @@
on the disk. A power failure in such a situation might lead to
irrecoverable data corruption. Administrators should try to ensure
that disks holding <productname>PostgreSQL</productname>'s
<acronym>WAL</acronym> log files do not make such false reports.
<acronym>WAL</acronym> files do not make such false reports.
(See <xref linkend="wal-reliability"/>.)
</para>
<para>
After a checkpoint has been made and the log flushed, the
After a checkpoint has been made and the WAL flushed, the
checkpoint's position is saved in the file
<filename>pg_control</filename>. Therefore, at the start of recovery,
the server first reads <filename>pg_control</filename> and
then the checkpoint record; then it performs the REDO operation by
scanning forward from the log location indicated in the checkpoint
scanning forward from the WAL location indicated in the checkpoint
record. Because the entire content of data pages is saved in the
log on the first page modification after a checkpoint (assuming
WAL on the first page modification after a checkpoint (assuming
<xref linkend="guc-full-page-writes"/> is not disabled), all pages
changed since the checkpoint will be restored to a consistent
state.
@@ -897,7 +897,7 @@
<para>
To deal with the case where <filename>pg_control</filename> is
corrupt, we should support the possibility of scanning existing log
corrupt, we should support the possibility of scanning existing WAL
segments in reverse order &mdash; newest to oldest &mdash; in order to find the
latest checkpoint. This has not been implemented yet.
<filename>pg_control</filename> is small enough (less than one disk page)