Hi everyone,
I've been playing with various wal_level settings, mainly to find out
what is the overhead of hot standby, and I've noticed a strange thing.
In some cases the wal_level=minimal produces signigicantly more xlog
data than wal_level=archive (and hot_standby).
=====================================================================
Example:
1) with wal_level=minimal
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
------------------------------------
(000000010000000000000001,5799664)
$ pgbench -i -s 10
NOTICE: table "pgbench_branches" does not exist, skipping
NOTICE: table "pgbench_tellers" does not exist, skipping
NOTICE: table "pgbench_accounts" does not exist, skipping
NOTICE: table "pgbench_history" does not exist, skipping
creating tables...
10000 tuples done.
20000 tuples done.
...
990000 tuples done.
1000000 tuples done.
set primary key...
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_branches_pkey" for table "pgbench_branches"
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_tellers_pkey" for table "pgbench_tellers"
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_accounts_pkey" for table "pgbench_accounts"
vacuum...done.
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
------------------------------------
(000000010000000000000001,5945832)
$ pgbench -c 1 -t 10000
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
query mode: simple
number of clients: 1
number of threads: 1
number of transactions per client: 10000
number of transactions actually processed: 10000/10000
tps = 515.508932 (including connections establishing)
tps = 515.623415 (excluding connections establishing)
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
------------------------------------
(000000010000000000000006,3395840)
=====================================================================
2) with wal_level=archive
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
------------------------------------
(000000010000000000000001,5799708)
$ pgbench -i -s 10
NOTICE: table "pgbench_branches" does not exist, skipping
NOTICE: table "pgbench_tellers" does not exist, skipping
NOTICE: table "pgbench_accounts" does not exist, skipping
NOTICE: table "pgbench_history" does not exist, skipping
creating tables...
10000 tuples done.
20000 tuples done.
...
990000 tuples done.
1000000 tuples done.
set primary key...
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_branches_pkey" for table "pgbench_branches"
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_tellers_pkey" for table "pgbench_tellers"
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index
"pgbench_accounts_pkey" for table "pgbench_accounts"
vacuum...done.
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
------------------------------------
(00000001000000000000000B,8772044)
$ pgbench -c 1 -t 10000
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
query mode: simple
number of clients: 1
number of threads: 1
number of transactions per client: 10000
number of transactions actually processed: 10000/10000
tps = 748.476327 (including connections establishing)
tps = 748.664607 (excluding connections establishing)
$ psql -c "select pg_xlogfile_name_offset(pg_current_xlog_location())"
pg_xlogfile_name_offset
-------------------------------------
(00000001000000000000000B,13937304)
=====================================================================
Which means the pgbench init takes about 142kB with 'minimal' level and
about 163MB with 'archive' level (which is expected).
But the actual pgbench run produces much more xlog data with minimal wal
level compared to archive level. With minimal level it produces about
90MB and with archive level it produces just about 5MB.
I've check the docs and the only possible explanation I've noticed when
reading the docs is that while wal_level=minimal allows to skip logging
of some bulk operations (e.g. COPY, which is exactly what pgbench does
when initializing the DB), it may need to log more data later (when
actually running the bench).
Is that a correct conclusion, or am I missing something?
regards
Tomas
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