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Date: Sat, 19 Apr 2008 16:06:05 +0100
From: James Mansion <james@mansionfamily.plus.com>
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To: Greg Smith <gsmith@gregsmith.com>
CC: Marinos Yannikos <mjy@geizhals.at>, pgsql-performance@postgresql.org
Subject: Re: Background writer underemphasized ...
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Greg Smith wrote:
> Using the background writer more assures that the cache on the 
> controller is going to be written to aggressively, so it may be 
> somewhat filled already come checkpoint time.  If you leave the writer 
> off, when the checkpoint comes you're much more likely to have the 
> full 2GB available to absorb a large block of writes.
But isn't it the case that while using background writer might result in 
*slightly* more data to write (since data that is updated several times 
might actually be sent several times), the total amount of data in both 
cases is much the same?  And if the buffer backed up in the BGW case, 
wouldn't it also back up (more?) if the writes are deferred?  And in 
fact by sending earlier, the real bottleneck (the disks) could have been 
getting on with it and staring their IO earlier?

Can you explian your reasoning a bit more?

James