Re: per backend WAL statistics - Mailing list pgsql-hackers

Subject: Clarification Needed on WAL Pending Checks in Patchset

Hi,

Thank you for the patchset. I’ve spent some time learning and reviewing it and have 2 comments.  I'm new to PostgreSQL hacking, so please bear with me if I make mistakes or say something that seems trivial.

I noticed that in patches prior to patch 6, the function pgstat_backend_wal_have_pending() was implemented as follows:

/* * To determine whether any WAL activity has occurred since last time, not * only the number of generated WAL records but also the numbers of WAL * writes and syncs need to be checked. Because even transactions that * generate no WAL records can write or sync WAL data when flushing the * data pages. */
static bool
pgstat_backend_wal_have_pending(void)
{    PgStat_PendingWalStats pending_wal;
    pending_wal = PendingBackendStats.pending_wal;
    return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||           pending_wal.wal_write != 0 || pending_wal.wal_sync != 0;
}

Starting with patch 7, it seems the implementation was simplified to:

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

Meanwhile, the cluster-level check in the function pgstat_wal_have_pending_cb() still performs the additional checks:

bool
pgstat_wal_have_pending_cb(void)
{    return pgWalUsage.wal_records != prevWalUsage.wal_records ||           PendingWalStats.wal_write != 0 ||           PendingWalStats.wal_sync != 0;
}

The difference lies in the removal of the checks for wal_write and wal_sync from the per-backend function. I assume that this may be due to factoring out these counters—perhaps because they can now be extracted from pg_stat_get_backend_io(). However, I’m not entirely sure I grasp the full rationale behind this change. 

Another one is on: