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Re: performance drop on RAID5

From: "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>
To: Alexandre Barros <alexandre(at)vectorx(dot)com(dot)br>
Cc: postgres performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: performance drop on RAID5
Date: 2005-08-24 16:17:42
Message-ID: 430C9DA6.2080801@commandprompt.com (view raw or flat)
Thread:
Lists: pgsql-performance
Alexandre Barros wrote:

> Hello,
> i have a pg-8.0.3 running on Linux  kernel  2.6.8,  CPU  Sempron 
> 2600+, 1Gb RAM on IDE HD ( which could be called a "heavy desktop" ), 
> measuring this performance with pgbench ( found on /contrib ) it gave 
> me an average ( after several runs ) of 170 transactions per second;

That is going to be because IDE drives LIE about write times because of 
the large cache.

> for the sake of experimentation ( actually, i'm scared this IDE drive 
> could fail at any time, hence i'm looking for an alternative, more 
> "robust", machine ), i've installed on an aging Compaq Proliant server 
> ( freshly compiled SMP kernel 2.6.12.5  with preemption ), dual 
> Pentium III Xeon 500Mhz, 512Mb RAM, (older) SCSI-2 80pin drives, and 
> re-tested, when the database was on a single SCSI drive, pgbench gave 
> me an average of 90 transactions per second, but, and that scared me 
> most, when the database was on a RAID-5 array ( four 9Gb disks, using 
> linux software RAID mdadm and LVM2, with the default filesystem 
> cluster size of 32Kb ), the performance dropped to about 55 
> transactions per second.


That seems more reasonable and probably truthful. I would be curious 
what type of performance you would get with the exact same
setup EXCEPT remove LVM2. Just have the software RAID. In fact, since 
you have 4 drives you could do RAID 10.

>
> i wouldn't be so stunned if the newer machine was ( say ) twice faster 
> than the older server, but over three times faster is disturbing.
>
> the postgresql.conf of both machines is here:
>
> max_connections = 50
> shared_buffers = 1000           # min 16, at least max_connections*2, 
> 8KB each

You should look at the annotated conf:

http://www.powerpostgresql.com/Downloads/annotated_conf_80.html

Sincerely,

Joshua D. Drake



> debug_print_parse = false
> debug_print_rewritten = false
> debug_print_plan = false
> debug_pretty_print = false
> log_statement = 'all'
> log_parser_stats        = false
> log_planner_stats       = false
> log_executor_stats      = false
> log_statement_stats     = false
> lc_messages = 'en_US'           # locale for system error message strings
> lc_monetary = 'en_US'           # locale for monetary formatting
> lc_numeric = 'en_US'            # locale for number formatting
> lc_time = 'en_US'               # locale for time formatting
>
> many thanks in advance !
>
>
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