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Message-ID: <4507DA84.2090509@designaproduct.biz>
Date: Wed, 13 Sep 2006 12:16:36 +0200
From: Laszlo Nagy <gandalf@designaproduct.biz>
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To: pgsql-performance@postgresql.org
Subject: Re: Poor performance on seq scan
References: <C12C8033.30BC3%llonergan@greenplum.com>
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> I have had extremely bad performance historically with onboard SATA chipsets
> on Linux.  The one exception has been with the Intel based chipsets (not the
> CPU, the I/O chipset).
>   
This board has Intel chipset. I cannot remember the exact type but it 
was not in the low end category.
dmesg says:

<Intel ICH7 SATA300 controller>
kernel: ad4: 152626MB <SAMSUNG HD160JJ ZM100-33> at ata2-master SATA150
kernel: ad4: 152627MB <SAMSUNG HD160JJ ZM100-33> at ata3-master SATA150

> It is very likely that you are having problems with the driver for the
> chipset.
>
> Are you running RAID1 in hardware?  If so, turn it off and see what the
> performance is.  The onboard hardware RAID is worse than useless, it
> actually slows the I/O down.
>   
I'm using software raid, namely gmirror:

GEOM_MIRROR: Device gm0 created (id=2574033628).
GEOM_MIRROR: Device gm0: provider ad4 detected.
GEOM_MIRROR: Device gm0: provider ad6 detected.
GEOM_MIRROR: Device gm0: provider ad4 activated.
GEOM_MIRROR: Device gm0: provider ad6 activated.

#gmirror list
Geom name: gm0
State: COMPLETE
Components: 2
Balance: round-robin
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 1
ID: 2574033628
Providers:
1. Name: mirror/gm0
   Mediasize: 160040803328 (149G)
   Sectorsize: 512
   Mode: r5w5e6
Consumers:
1. Name: ad4
   Mediasize: 160040803840 (149G)
   Sectorsize: 512
   Mode: r1w1e1
   State: ACTIVE
   Priority: 0
   Flags: DIRTY
   GenID: 0
   SyncID: 1
   ID: 1153981856
2. Name: ad6
   Mediasize: 160041885696 (149G)
   Sectorsize: 512
   Mode: r1w1e1
   State: ACTIVE
   Priority: 0
   Flags: DIRTY
   GenID: 0
   SyncID: 1
   ID: 3520427571


I tried to do:

#sysctl vfs.read_max=32
vfs.read_max: 6 -> 32

but I could not reach better disk read performance.

Thank you for your suggestions. Looks like I need to buy SCSI disks.

Regards,

   Laszlo