Re: [Patch] New pg_stat_tablespace view

Hi Ahmed,

Thanks for reviewing my patch!

> I am having a problem building the v5 patch locally. I get the following
compile errors:
You are right: 3f2f5e7c4c moved those counters to
PgStat_TableCountsXact and my v5 missed it. Fixed in v6.

While testing that fix I found a worse bug, which v6 also fixes.

The buffer manager reports block I/O timings from the checkpointer and the
background writer too. Those two processes never call pgstat_report_stat(),
so the pending entry they created was never flushed. That caused two
problems.

First, an unflushed pending entry keeps its shared entry alive, because
pgstat_gc_entry_refs() skips refs that still have pending data. So after
DROP TABLESPACE the entry could not be freed, and the checkpointer crashed
while writing the stats file at shutdown:

TRAP: failed Assert("!ps->dropped"), File: "pgstat.c", Line: 1776
LOG: checkpointer process (PID ...) was terminated by signal 6

To reproduce, on an assert build with track_io_timing=on: create a
tablespace, insert enough rows that the checkpointer writes to it,
CHECKPOINT, DROP TABLE, DROP TABLESPACE, then pg_ctl stop -m fast.

Second, blk_write_time stayed at zero while the server ran. In one test the
checkpointer wrote 161 buffers in 3 ms and the view did not move at all.
The value only appeared after a restart. Most writes come from the
checkpointer, so the column was close to useless.

v6 keeps these timings in process-local memory and flushes them through
flush_static_cb, the same way PGSTAT_KIND_BACKEND handles its own data.
Backends flush through pgstat_report_stat(). The checkpointer and the
background writer call pgstat_flush_tablespace_times() from
pgstat_report_checkpointer() and pgstat_report_bgwriter().

v6 also adds pgstat_create_tablespace(), called from CreateTableSpace().
v5 had a drop but no create, and this clears old stats if a tablespace OID
is ever reused.

Thanks,

Shihao