From: Mladen Gogala <mgogala@vmsinfo.com>
To: Jon Nelson <jnelson+pgsql@jamponi.net>
Cc: Mladen Gogala <mladen.gogala@vmsinfo.com>
Cc: david@lang.hm <david@lang.hm>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: Vitalii Tymchyshyn <tivv00@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Slow count(*) again...
Date: Tue, 12 Oct 2010 09:55:39 -0400
Message-ID: <4CB468DB.8040902@vmsinfo.com> (raw)
In-Reply-To: <AANLkTina3zfQ01UwPqps_hogvR-ge=ur9zHCSsxvKAm8@mail.gmail.com>
References: <201010091626.18636.neil.whelchel@gmail.com>
<AANLkTimayTM__hHu9RoiLaNhzfpNY6Vo5y4OkbX_sV=o@mail.gmail.com>
<4CB34D69.6060108@emolecules.com>
<4CB4149D.8030803@gmail.com>
<4CB41902.1020503@postnewspapers.com.au>
<alpine.DEB.2.00.1010120118030.21889@asgard.lang.hm>
<4CB4542E.5010406@vmsinfo.com>
<AANLkTina3zfQ01UwPqps_hogvR-ge=ur9zHCSsxvKAm8@mail.gmail.com>
Jon Nelson wrote:
>
> Well, I didn't quite mean that - having no familiarity with Oracle I
> don't know what the alter system statement does, but I was talking
> specifically about the linux buffer and page cache.
>
Those are not utilized by Oracle. This is a RAC instance, running on
top of ASM, which is an Oracle volume manager, using raw devices. There
is no file system on those disks:
SQL> select file_name from dba_data_files
2 where tablespace_name='ADBASE_DATA';
FILE_NAME
--------------------------------------------------------------------------------
+DGDATA/stag3/datafile/adbase_data.262.727278257
+DGDATA/stag3/datafile/adbase_data.263.727278741
+DGDATA/stag3/datafile/adbase_data.264.727280145
+DGDATA/stag3/datafile/adbase_data.265.727280683
[oracle@lpo-oracle-30 ~]$ $ORA_CRS_HOME/bin/crs_stat -l
NAME=ora.STAG3.STAG31.inst
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.STAG3.STAG32.inst
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.STAG3.db
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.lpo-oracle-30.ASM1.asm
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.lpo-oracle-30.LISTENER_LPO-ORACLE-30.lsnr
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.lpo-oracle-30.gsd
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.lpo-oracle-30.ons
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.lpo-oracle-30.vip
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-30
NAME=ora.lpo-oracle-31.ASM2.asm
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.lpo-oracle-31.LISTENER_LPO-ORACLE-31.lsnr
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.lpo-oracle-31.gsd
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.lpo-oracle-31.ons
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
NAME=ora.lpo-oracle-31.vip
TYPE=application
TARGET=ONLINE
STATE=ONLINE on lpo-oracle-31
The only way to flush cache is the aforementioned "alter system"
command. AFAIK, Postgres doesn't have anything like that. Oracle uses
raw devices precisely to avoid double buffering.
--
Mladen Gogala
Sr. Oracle DBA
1500 Broadway
New York, NY 10036
(212) 329-5251
http://www.vmsinfo.com
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Subject: Re: Slow count(*) again...
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