Message-ID: <42A47129.30309@arbash-meinel.com>
Date: Mon, 06 Jun 2005 10:52:09 -0500
From: John A Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.2-1.3.3 (X11/20050513)
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To: Michael Stone <mstone+postgres@mathom.us>
Cc: Postgresql Performance <pgsql-performance@postgresql.org>
Subject: Re: Performance nightmare with dspam (urgent) (resolved)
References: <200506012019.13218.cshobe@seattleserver.com>
	<200506061454.48018.lists@seattleserver.com>
	<42A466E7.4080808@arbash-meinel.com>
	<20050606151911.GB19670@mathom.us>
In-Reply-To: <20050606151911.GB19670@mathom.us>
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Michael Stone wrote:
> On Mon, Jun 06, 2005 at 10:08:23AM -0500, John A Meinel wrote:
> 
>> I don't know if you can do it, but it would be nice to see this be 1
>> RAID1 for OS, 1 RAID10 for pg_xlog, 
> 
> 
> That's probably overkill--it's a relatively small sequential-write
> partition with really small writes; I don't see how pg_xlog would
> benefit from raid10 as opposed to raid1.
> Mike Stone
> 

pg_xlog benefits from being super fast. Because it has to be fully
synced before the rest of the data can be committed. Yes they are small,
but if you can make it fast, you eliminate that overhead. It also
benefits from having it's own spindle, because you eliminate the seek
time. (Since it is always appending)

Anyway, my point is that pg_xlog isn't necessarily tiny. Many people
seem to set it as high as 100-200, and each one is 16MB.

But one other thing to consider is to make pg_xlog on a battery backed
ramdisk. Because it really *can* use the extra speed. I can't say that a
ramdisk is more cost effective than faster db disks. But if you aren't
using many checkpoint_segments, it seems like you could get a 1GB
ramdisk, and probably have a pretty good performance boost. (I have not
tested this personally, though).

Since he is using the default settings (mostly) for dspam, he could
probably get away with something like a 256MB ramdisk.

The only prices I could find with a few minutes of googleing was:
http://www.cenatek.com/store/category.cfm?Category=15
Which is $1.6k for 2GB.

But there is also a product that is being developed, which claims $60
for the PCI card, you supply the memory. It has 4 DDR slots
http://www.engadget.com/entry/1234000227045399/
And you can get a 128MB SDRAM ECC module for around $22
http://www.newegg.com/Product/Product.asp?Item=N82E16820998004
So that would put the total cost of a 512MB battery backed ramdisk at
$60 + 4*22 = $150.

That certainly seems less than what you would pay for the same speed in
hard-drives.
Unfortunately the Giga-byte iRam seems to just be in the demo stage. But
if they aren't lying in the press releases, it would certainly be
something to keep an eye on.

John
=:->

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