SSL pgstat_wal.c

/* -------------------------------------------------------------------------
 *
 * pgstat_wal.c
 *   Implementation of WAL statistics.
 *
 * This file contains the implementation of WAL statistics. It is kept
 * separate from pgstat.c to enforce the line between the statistics access /
 * storage implementation and the details about individual types of
 * statistics.
 *
 * Copyright (c) 2001-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *   src/backend/utils/activity/pgstat_wal.c
 * -------------------------------------------------------------------------
 */

#include "postgres.h"

#include "executor/instrument.h"
#include "utils/pgstat_internal.h"


/*
 * WAL usage counters saved from pgWalUsage at the previous call to
 * pgstat_report_wal(). This is used to calculate how much WAL usage
 * happens between pgstat_report_wal() calls, by subtracting
 * the previous counters from the current ones.
 */
static WalUsage prevWalUsage;


/*
 * Calculate how much WAL usage counters have increased and update
 * shared WAL and IO statistics.
 *
 * Must be called by processes that generate WAL, that do not call
 * pgstat_report_stat(), like walwriter.
 *
 * "force" set to true ensures that the statistics are flushed; note that
 * this needs to acquire the pgstat shmem LWLock, waiting on it.  When
 * set to false, the statistics may not be flushed if the lock could not
 * be acquired.
 */
void
pgstat_report_wal(bool force)
{
 bool  nowait;

 /* like in pgstat.c, don't wait for lock acquisition when !force */
 nowait = !force;

 /* flush wal stats */
 (void) pgstat_wal_flush_cb(nowait);
 pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);

 /* flush IO stats */
 pgstat_flush_io(nowait);
 (void) pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_IO);
}

/*
 * Support function for the SQL-callable pgstat* functions. Returns
 * a pointer to the WAL statistics struct.
 */
PgStat_WalStats *
pgstat_fetch_stat_wal(void)
{
 pgstat_snapshot_fixed(PGSTAT_KIND_WAL);

 return &pgStatLocal.snapshot.wal;
}

/*
 * To determine whether WAL usage happened.
 */
static inline bool
pgstat_wal_have_pending(void)
{
 return pgWalUsage.wal_records != prevWalUsage.wal_records;
}

/*
 * Calculate how much WAL usage counters have increased by subtracting the
 * previous counters from the current ones.
 *
 * If nowait is true, this function returns true if the lock could not be
 * acquired. Otherwise return false.
 */
bool
pgstat_wal_flush_cb(bool nowait)
{
 PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
 WalUsage wal_usage_diff = {0};

 Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
 Assert(pgStatLocal.shmem != NULL &&
     !pgStatLocal.shmem->is_shutdown);

 /*
  * This function can be called even if nothing at all has happened. Avoid
  * taking lock for nothing in that case.
 */
 if (!pgstat_wal_have_pending())
  return false;

 /*
  * We don't update the WAL usage portion of the local WalStats elsewhere.
  * Calculate how much WAL usage counters were increased by subtracting the
  * previous counters from the current ones.
 */
 WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevWalUsage);

 if (!nowait)
  LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
 else if (!LWLockConditionalAcquire(&stats_shmem->lock, LW_EXCLUSIVE))
  return true;

#define WALSTAT_ACC(fld, var_to_add) \
 (stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 WALSTAT_ACC(wal_records, wal_usage_diff);
 WALSTAT_ACC(wal_fpi, wal_usage_diff);
 WALSTAT_ACC(wal_bytes, wal_usage_diff);
 WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
#undef WALSTAT_ACC

 LWLockRelease(&stats_shmem->lock);

 /*
  * Save the current counters for the subsequent calculation of WAL usage.
 */
 prevWalUsage = pgWalUsage;

 return false;
}

void
pgstat_wal_init_backend_cb(void)
{
 /*
  * Initialize prevWalUsage with pgWalUsage so that pgstat_wal_flush_cb()
  * can calculate how much pgWalUsage counters are increased by subtracting
  * prevWalUsage from pgWalUsage.
 */
 prevWalUsage = pgWalUsage;
}

void
pgstat_wal_init_shmem_cb(void *stats)
{
 PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *) stats;

 LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
}

void
pgstat_wal_reset_all_cb(TimestampTz ts)
{
 PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;

 LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
 memset(&stats_shmem->stats, 0, sizeof(stats_shmem->stats));
 stats_shmem->stats.stat_reset_timestamp = ts;
 LWLockRelease(&stats_shmem->lock);
}

void
pgstat_wal_snapshot_cb(void)
{
 PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;

 LWLockAcquire(&stats_shmem->lock, LW_SHARED);
 memcpy(&pgStatLocal.snapshot.wal, &stats_shmem->stats,
     sizeof(pgStatLocal.snapshot.wal));
 LWLockRelease(&stats_shmem->lock);
}