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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