<para>
The server's checkpointer process automatically performs
a checkpoint every so often. A checkpoint is begun every <xref
- linkend="guc-checkpoint-segments"> log segments, or every <xref
- linkend="guc-checkpoint-timeout"> seconds, whichever comes first.
- The default settings are 3 segments and 300 seconds (5 minutes), respectively.
+ linkend="guc-checkpoint-timeout"> seconds, or if
+ <xref linkend="guc-max-wal-size"> is about to be exceeded,
+ whichever comes first.
+ The default settings are 5 minutes and 128 MB, respectively.
If no WAL has been written since the previous checkpoint, new checkpoints
will be skipped even if <varname>checkpoint_timeout</> has passed.
(If WAL archiving is being used and you want to put a lower limit on how
</para>
<para>
- Reducing <varname>checkpoint_segments</varname> and/or
- <varname>checkpoint_timeout</varname> causes checkpoints to occur
+ Reducing <varname>checkpoint_timeout</varname> and/or
+ <varname>max_wal_size</varname> causes checkpoints to occur
more often. This allows faster after-crash recovery, since less work
will need to be redone. However, one must balance this against the
increased cost of flushing dirty data pages more often. If
parameter. If checkpoints happen closer together than
<varname>checkpoint_warning</> seconds,
a message will be output to the server log recommending increasing
- <varname>checkpoint_segments</varname>. Occasional appearance of such
+ <varname>max_wal_size</varname>. Occasional appearance of such
a message is not cause for alarm, but if it appears often then the
checkpoint control parameters should be increased. Bulk operations such
as large <command>COPY</> transfers might cause a number of such warnings
- to appear if you have not set <varname>checkpoint_segments</> high
+ to appear if you have not set <varname>max_wal_size</> high
enough.
</para>
<xref linkend="guc-checkpoint-completion-target">, which is
given as a fraction of the checkpoint interval.
The I/O rate is adjusted so that the checkpoint finishes when the
- given fraction of <varname>checkpoint_segments</varname> WAL segments
- have been consumed since checkpoint start, or the given fraction of
- <varname>checkpoint_timeout</varname> seconds have elapsed,
- whichever is sooner. With the default value of 0.5,
+ given fraction of
+ <varname>checkpoint_timeout</varname> seconds have elapsed, or before
+ <varname>max_wal_size</varname> is exceeded, whichever is sooner.
+ With the default value of 0.5,
<productname>PostgreSQL</> can be expected to complete each checkpoint
in about half the time before the next checkpoint starts. On a system
that's very close to maximum I/O throughput during normal operation,
</para>
<para>
- There will always be at least one WAL segment file, and will normally
- not be more than (2 + <varname>checkpoint_completion_target</varname>) * <varname>checkpoint_segments</varname> + 1
- or <varname>checkpoint_segments</> + <xref linkend="guc-wal-keep-segments"> + 1
- files. Each segment file is normally 16 MB (though this size can be
- altered when building the server). You can use this to estimate space
- requirements for <acronym>WAL</acronym>.
- Ordinarily, when old log segment files are no longer needed, they
- are recycled (that is, renamed to become future segments in the numbered
- sequence). If, due to a short-term peak of log output rate, there
- are more than 3 * <varname>checkpoint_segments</varname> + 1
- segment files, the unneeded segment files will be deleted instead
- of recycled until the system gets back under this limit.
+ The number of WAL segment files in <filename>pg_xlog</> directory depends on
+ <varname>min_wal_size</>, <varname>max_wal_size</> and
+ the amount of WAL generated in previous checkpoint cycles. When old log
+ 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</> 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
+ removes the rest. The estimate is based on a moving average of the number
+ of WAL files used in previous checkpoint cycles. The moving average
+ is increased immediately if the actual usage exceeds the estimate, so it
+ accommodates peak usage rather average usage to some extent.
+ <varname>min_wal_size</> puts a minimum on the amount of WAL files
+ recycled for future usage; that much WAL is always recycled for future use,
+ even if the system is idle and the WAL usage estimate suggests that little
+ WAL is needed.
+ </para>
+
+ <para>
+ Independently of <varname>max_wal_size</varname>,
+ <xref linkend="guc-wal-keep-segments"> + 1 most recent WAL files are
+ kept at all times. Also, if WAL archiving is used, old segments can not be
+ removed or recycled until they are archived. If WAL archiving cannot keep up
+ with the pace that WAL is generated, or if <varname>archive_command</varname>
+ fails repeatedly, old WAL files will accumulate in <filename>pg_xlog</>
+ until the situation is resolved. A slow or failed standby server that
+ uses a replication slot will have the same effect (see
+ <xref linkend="streaming-replication-slots">).
</para>
<para>
master because restartpoints can only be performed at checkpoint records.
A restartpoint is triggered when a checkpoint record is reached if at
least <varname>checkpoint_timeout</> seconds have passed since the last
- restartpoint. In standby mode, a restartpoint is also triggered if at
- least <varname>checkpoint_segments</> log segments have been replayed
- since the last restartpoint.
+ restartpoint, or if WAL size is about to exceed
+ <varname>max_wal_size</>.
</para>
<para>
/* User-settable parameters */
-int CheckPointSegments = 3;
+int max_wal_size = 8; /* 128 MB */
+int min_wal_size = 5; /* 80 MB */
int wal_keep_segments = 0;
int XLOGbuffers = -1;
int XLogArchiveTimeout = 0;
#define NUM_XLOGINSERT_LOCKS 8
/*
- * XLOGfileslop is the maximum number of preallocated future XLOG segments.
- * When we are done with an old XLOG segment file, we will recycle it as a
- * future XLOG segment as long as there aren't already XLOGfileslop future
- * segments; else we'll delete it. This could be made a separate GUC
- * variable, but at present I think it's sufficient to hardwire it as
- * 2*CheckPointSegments+1. Under normal conditions, a checkpoint will free
- * no more than 2*CheckPointSegments log segments, and we want to recycle all
- * of them; the +1 allows boundary cases to happen without wasting a
- * delete/create-segment cycle.
+ * Max distance from last checkpoint, before triggering a new xlog-based
+ * checkpoint.
*/
-#define XLOGfileslop (2*CheckPointSegments + 1)
+int CheckPointSegments;
+/* Estimated distance between checkpoints, in bytes */
+static double CheckPointDistanceEstimate = 0;
+static double PrevCheckPointDistance = 0;
/*
* GUC support
static bool XLogCheckpointNeeded(XLogSegNo new_segno);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
- bool find_free, int *max_advance,
+ bool find_free, XLogSegNo max_segno,
bool use_lock);
static int XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
int source, bool notexistOk);
static int emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
-static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr);
+static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr PriorRedoPtr, XLogRecPtr endptr);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
#endif
}
+/*
+ * Calculate CheckPointSegments based on max_wal_size and
+ * checkpoint_completion_target.
+ */
+static void
+CalculateCheckpointSegments(void)
+{
+ double target;
+
+ /*-------
+ * Calculate the distance at which to trigger a checkpoint, to avoid
+ * exceeding max_wal_size. This is based on two assumptions:
+ *
+ * a) we keep WAL for two checkpoint cycles, back to the "prev" checkpoint.
+ * b) during checkpoint, we consume checkpoint_completion_target *
+ * number of segments consumed between checkpoints.
+ *-------
+ */
+ target = (double ) max_wal_size / (2.0 + CheckPointCompletionTarget);
+
+ /* round down */
+ CheckPointSegments = (int) target;
+
+ if (CheckPointSegments < 1)
+ CheckPointSegments = 1;
+}
+
+void
+assign_max_wal_size(int newval, void *extra)
+{
+ max_wal_size = newval;
+ CalculateCheckpointSegments();
+}
+
+void
+assign_checkpoint_completion_target(double newval, void *extra)
+{
+ CheckPointCompletionTarget = newval;
+ CalculateCheckpointSegments();
+}
+
+/*
+ * At a checkpoint, how many WAL segments to recycle as preallocated future
+ * XLOG segments? Returns the highest segment that should be preallocated.
+ */
+static XLogSegNo
+XLOGfileslop(XLogRecPtr PriorRedoPtr)
+{
+ XLogSegNo minSegNo;
+ XLogSegNo maxSegNo;
+ double distance;
+ XLogSegNo recycleSegNo;
+
+ /*
+ * Calculate the segment numbers that min_wal_size and max_wal_size
+ * correspond to. Always recycle enough segments to meet the minimum, and
+ * remove enough segments to stay below the maximum.
+ */
+ minSegNo = PriorRedoPtr / XLOG_SEG_SIZE + min_wal_size - 1;
+ maxSegNo = PriorRedoPtr / XLOG_SEG_SIZE + max_wal_size - 1;
+
+ /*
+ * Between those limits, recycle enough segments to get us through to the
+ * estimated end of next checkpoint.
+ *
+ * To estimate where the next checkpoint will finish, assume that the
+ * system runs steadily consuming CheckPointDistanceEstimate
+ * bytes between every checkpoint.
+ *
+ * The reason this calculation is done from the prior checkpoint, not the
+ * one that just finished, is that this behaves better if some checkpoint
+ * cycles are abnormally short, like if you perform a manual checkpoint
+ * right after a timed one. The manual checkpoint will make almost a full
+ * cycle's worth of WAL segments available for recycling, because the
+ * segments from the prior's prior, fully-sized checkpoint cycle are no
+ * longer needed. However, the next checkpoint will make only few segments
+ * available for recycling, the ones generated between the timed
+ * checkpoint and the manual one right after that. If at the manual
+ * checkpoint we only retained enough segments to get us to the next timed
+ * one, and removed the rest, then at the next checkpoint we would not
+ * have enough segments around for recycling, to get us to the checkpoint
+ * after that. Basing the calculations on the distance from the prior redo
+ * pointer largely fixes that problem.
+ */
+ distance = (2.0 + CheckPointCompletionTarget) * CheckPointDistanceEstimate;
+ /* add 10% for good measure. */
+ distance *= 1.10;
+
+ recycleSegNo = (XLogSegNo) ceil(((double) PriorRedoPtr + distance) / XLOG_SEG_SIZE);
+
+ if (recycleSegNo < minSegNo)
+ recycleSegNo = minSegNo;
+ if (recycleSegNo > maxSegNo)
+ recycleSegNo = maxSegNo;
+
+ return recycleSegNo;
+}
+
/*
* Check whether we've consumed enough xlog space that a checkpoint is needed.
*
char zbuffer_raw[XLOG_BLCKSZ + MAXIMUM_ALIGNOF];
char *zbuffer;
XLogSegNo installed_segno;
- int max_advance;
+ XLogSegNo max_segno;
int fd;
int nbytes;
* pre-create a future log segment.
*/
installed_segno = logsegno;
- max_advance = XLOGfileslop;
+
+ /*
+ * XXX: What should we use as max_segno? We used to use XLOGfileslop when
+ * that was a constant, but that was always a bit dubious: normally, at a
+ * checkpoint, XLOGfileslop was the offset from the checkpoint record,
+ * but here, it was the offset from the insert location. We can't do the
+ * normal XLOGfileslop calculation here because we don't have access to
+ * the prior checkpoint's redo location. So somewhat arbitrarily, just
+ * use CheckPointSegments.
+ */
+ max_segno = logsegno + CheckPointSegments;
if (!InstallXLogFileSegment(&installed_segno, tmppath,
- *use_existent, &max_advance,
+ *use_existent, max_segno,
use_lock))
{
/*
/*
* Now move the segment into place with its final name.
*/
- if (!InstallXLogFileSegment(&destsegno, tmppath, false, NULL, false))
+ if (!InstallXLogFileSegment(&destsegno, tmppath, false, 0, false))
elog(ERROR, "InstallXLogFileSegment should not have failed");
}
* number at or after the passed numbers. If FALSE, install the new segment
* exactly where specified, deleting any existing segment file there.
*
- * *max_advance: maximum number of segno slots to advance past the starting
- * point. Fail if no free slot is found in this range. On return, reduced
- * by the number of slots skipped over. (Irrelevant, and may be NULL,
- * when find_free is FALSE.)
+ * max_segno: maximum segment number to install the new file as. Fail if no
+ * free slot is found between *segno and max_segno. (Ignored when find_free
+ * is FALSE.)
*
* use_lock: if TRUE, acquire ControlFileLock while moving file into
* place. This should be TRUE except during bootstrap log creation. The
* caller must *not* hold the lock at call.
*
* Returns TRUE if the file was installed successfully. FALSE indicates that
- * max_advance limit was exceeded, or an error occurred while renaming the
+ * max_segno limit was exceeded, or an error occurred while renaming the
* file into place.
*/
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
- bool find_free, int *max_advance,
+ bool find_free, XLogSegNo max_segno,
bool use_lock)
{
char path[MAXPGPATH];
/* Find a free slot to put it in */
while (stat(path, &stat_buf) == 0)
{
- if (*max_advance <= 0)
+ if ((*segno) >= max_segno)
{
/* Failed to find a free slot within specified range */
if (use_lock)
return false;
}
(*segno)++;
- (*max_advance)--;
XLogFilePath(path, ThisTimeLineID, *segno);
}
}
/*
* Recycle or remove all log files older or equal to passed segno
*
- * endptr is current (or recent) end of xlog; this is used to determine
+ * endptr is current (or recent) end of xlog, and PriorRedoRecPtr is the
+ * redo pointer of the previous checkpoint. These are used to determine
* whether we want to recycle rather than delete no-longer-wanted log files.
*/
static void
-RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr)
+RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr PriorRedoPtr, XLogRecPtr endptr)
{
XLogSegNo endlogSegNo;
- int max_advance;
+ XLogSegNo recycleSegNo;
DIR *xldir;
struct dirent *xlde;
char lastoff[MAXFNAMELEN];
struct stat statbuf;
/*
- * Initialize info about where to try to recycle to. We allow recycling
- * segments up to XLOGfileslop segments beyond the current XLOG location.
+ * Initialize info about where to try to recycle to.
*/
XLByteToPrevSeg(endptr, endlogSegNo);
- max_advance = XLOGfileslop;
+ recycleSegNo = XLOGfileslop(PriorRedoPtr);
xldir = AllocateDir(XLOGDIR);
if (xldir == NULL)
* for example can create symbolic links pointing to a
* separate archive directory.
*/
- if (lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
+ if (endlogSegNo <= recycleSegNo &&
+ lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
InstallXLogFileSegment(&endlogSegNo, path,
- true, &max_advance, true))
+ true, recycleSegNo, true))
{
ereport(DEBUG2,
(errmsg("recycled transaction log file \"%s\"",
xlde->d_name)));
CheckpointStats.ckpt_segs_recycled++;
/* Needn't recheck that slot on future iterations */
- if (max_advance > 0)
- {
- endlogSegNo++;
- max_advance--;
- }
+ endlogSegNo++;
}
else
{
elog(LOG, "%s complete: wrote %d buffers (%.1f%%); "
"%d transaction log file(s) added, %d removed, %d recycled; "
"write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
- "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
+ "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
+ "distance=%d kB, estimate=%d kB",
restartpoint ? "restartpoint" : "checkpoint",
CheckpointStats.ckpt_bufs_written,
(double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
total_secs, total_usecs / 1000,
CheckpointStats.ckpt_sync_rels,
longest_secs, longest_usecs / 1000,
- average_secs, average_usecs / 1000);
+ average_secs, average_usecs / 1000,
+ (int) (PrevCheckPointDistance / 1024.0),
+ (int) (CheckPointDistanceEstimate / 1024.0));
+}
+
+/*
+ * Update the estimate of distance between checkpoints.
+ *
+ * The estimate is used to calculate the number of WAL segments to keep
+ * preallocated, see XLOGFileSlop().
+ */
+static void
+UpdateCheckPointDistanceEstimate(uint64 nbytes)
+{
+ /*
+ * To estimate the number of segments consumed between checkpoints, keep
+ * a moving average of the amount of WAL generated in previous checkpoint
+ * cycles. However, if the load is bursty, with quiet periods and busy
+ * periods, we want to cater for the peak load. So instead of a plain
+ * moving average, let the average decline slowly if the previous cycle
+ * used less WAL than estimated, but bump it up immediately if it used
+ * more.
+ *
+ * When checkpoints are triggered by max_wal_size, this should converge to
+ * CheckpointSegments * XLOG_SEG_SIZE,
+ *
+ * Note: This doesn't pay any attention to what caused the checkpoint.
+ * Checkpoints triggered manually with CHECKPOINT command, or by e.g.
+ * starting a base backup, are counted the same as those created
+ * automatically. The slow-decline will largely mask them out, if they are
+ * not frequent. If they are frequent, it seems reasonable to count them
+ * in as any others; if you issue a manual checkpoint every 5 minutes and
+ * never let a timed checkpoint happen, it makes sense to base the
+ * preallocation on that 5 minute interval rather than whatever
+ * checkpoint_timeout is set to.
+ */
+ PrevCheckPointDistance = nbytes;
+ if (CheckPointDistanceEstimate < nbytes)
+ CheckPointDistanceEstimate = nbytes;
+ else
+ CheckPointDistanceEstimate =
+ (0.90 * CheckPointDistanceEstimate + 0.10 * (double) nbytes);
}
/*
XLogRecPtr recptr;
XLogCtlInsert *Insert = &XLogCtl->Insert;
uint32 freespace;
- XLogSegNo _logSegNo;
+ XLogRecPtr PriorRedoPtr;
XLogRecPtr curInsert;
VirtualTransactionId *vxids;
int nvxids;
(errmsg("concurrent transaction log activity while database system is shutting down")));
/*
- * Select point at which we can truncate the log, which we base on the
- * prior checkpoint's earliest info.
+ * Remember the prior checkpoint's redo pointer, used later to determine
+ * the point where the log can be truncated.
*/
- XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);
+ PriorRedoPtr = ControlFile->checkPointCopy.redo;
/*
* Update the control file.
* Delete old log files (those no longer needed even for previous
* checkpoint or the standbys in XLOG streaming).
*/
- if (_logSegNo)
+ if (PriorRedoPtr != InvalidXLogRecPtr)
{
+ XLogSegNo _logSegNo;
+
+ /* Update the average distance between checkpoints. */
+ UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);
+
+ XLByteToSeg(PriorRedoPtr, _logSegNo);
KeepLogSeg(recptr, &_logSegNo);
_logSegNo--;
- RemoveOldXlogFiles(_logSegNo, recptr);
+ RemoveOldXlogFiles(_logSegNo, PriorRedoPtr, recptr);
}
/*
{
XLogRecPtr lastCheckPointRecPtr;
CheckPoint lastCheckPoint;
- XLogSegNo _logSegNo;
+ XLogRecPtr PriorRedoPtr;
TimestampTz xtime;
/*
/*
* Update the shared RedoRecPtr so that the startup process can calculate
* the number of segments replayed since last restartpoint, and request a
- * restartpoint if it exceeds checkpoint_segments.
+ * restartpoint if it exceeds CheckPointSegments.
*
* Like in CreateCheckPoint(), hold off insertions to update it, although
* during recovery this is just pro forma, because no WAL insertions are
* happening.
*/
WALInsertLockAcquireExclusive();
- XLogCtl->Insert.RedoRecPtr = lastCheckPoint.redo;
+ RedoRecPtr = XLogCtl->Insert.RedoRecPtr = lastCheckPoint.redo;
WALInsertLockRelease();
/* Also update the info_lck-protected copy */
CheckPointGuts(lastCheckPoint.redo, flags);
/*
- * Select point at which we can truncate the xlog, which we base on the
- * prior checkpoint's earliest info.
+ * Remember the prior checkpoint's redo pointer, used later to determine
+ * the point at which we can truncate the log.
*/
- XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);
+ PriorRedoPtr = ControlFile->checkPointCopy.redo;
/*
* Update pg_control, using current time. Check that it still shows
* checkpoint/restartpoint) to prevent the disk holding the xlog from
* growing full.
*/
- if (_logSegNo)
+ if (PriorRedoPtr != InvalidXLogRecPtr)
{
XLogRecPtr receivePtr;
XLogRecPtr replayPtr;
TimeLineID replayTLI;
XLogRecPtr endptr;
+ XLogSegNo _logSegNo;
+
+ /* Update the average distance between checkpoints/restartpoints. */
+ UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);
+
+ XLByteToSeg(PriorRedoPtr, _logSegNo);
/*
* Get the current end of xlog replayed or received, whichever is
if (RecoveryInProgress())
ThisTimeLineID = replayTLI;
- RemoveOldXlogFiles(_logSegNo, endptr);
+ RemoveOldXlogFiles(_logSegNo, PriorRedoPtr, endptr);
/*
* Make more log segments if needed. (Do this after recycling old log
projects / postgresql / commitdiff