Re: performance drop on RAID5

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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