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"full_page_writes" makes no difference?

From: Tian Luo <jackrobin(at)gmail(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: "full_page_writes" makes no difference?
Date: 2011-05-04 04:17:02
Message-ID: 4DC0D33E.4060902@gmail.com (view raw or flat)
Thread:
Lists: pgsql-general
Hi guys,

No matter I turn on or turn off the "full_page_writes", I always
observe 8192-byte writes of log data for simple write operations
(write/update).

But according to the document, when this is off, it could speed up
operations but may cause problems during recovery. So, I guess this is
because it writes less when the option is turned off. However, this
contradicts my observations ....

If I am not missing anything, I find that the writes of log data go
through function "XLogWrite" in source file
"backend/access/transam/xlog.c".

In this file, log data are written with the following code:

from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
nbytes = npages * (Size) XLOG_BLCKSZ;
if (write(openLogFile, from, nbytes) != nbytes)
{
  ...
}

So, "nbytes" should always be multiples of XLOG_BLCKSZ, which in the
default case, is 8192.

My question is, if it always writes full pages no matter
"full_page_writes" is on or off, what is the difference?

Thanks!

Regards,
- Tian


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