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shared_buffers and shmmax

From: dx k9 <bitsandbytes88(at)hotmail(dot)com>
To: posgres support <pgsql-admin(at)postgresql(dot)org>
Subject: shared_buffers and shmmax
Date: 2008-07-22 13:39:57
Message-ID: BLU144-W43110EE920E5B083034F47D1850@phx.gbl (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-docspgsql-hackers
Hi,
I'm trying to understand what the documentation means by bytes per increment, what is the increment supposed to be bytes, MB, or Kb.  I have my shared_buffers set to 577 MB(4 instances) and I'm multiplying by 8400 bytes.  I would think I would want to keep everything in bytes and not mulitply bytes times MB, but this is what table 17-2 implies.  If I convert 577 to bytes and multiply, my calculator goes exponential on me. I'm going through this table and adding up to see what my shmmax should be (it's 7.5 GB) out of a total memory of 16 GB with 1000 max_connections right now.  What should I use as the "increment" value in regards to shared buffers, 577, 590848 or 605028352 ? 
 
a) 577 MB (This seems too small)
b) 590,848 Kb (this seems just right)
c) 605,028,352 bytes  (this seems too big, I hope it's not c)
 
Thanks,
~DjK
 
Table 17-2. Configuration parameters affecting PostgreSQL's shared memory usage







Name
Approximate multiplier (bytes per increment) as of 8.3


max_connections
1800 + 270 * max_locks_per_transaction

autovacuum_max_workers
1800 + 270 * max_locks_per_transaction

max_prepared_transactions
770 + 270 * max_locks_per_transaction

shared_buffers
8400 (assuming 8 kB BLCKSZ)

wal_buffers
8200 (assuming 8 kB XLOG_BLCKSZ)

max_fsm_relations
70

max_fsm_pages
6

Fixed space requirements
770 kB
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