Re: fix use of posix_fadvise in xlog.c

On Jun 9, 2010, at 11:25 PM, Heikki Linnakangas <heikki.linnakangas@enterprisedb.com > wrote:

> On 10/06/10 06:47, Mark Wong wrote:
>> I wanted to propose a fix for to xlog.c regarding the use of
>> posix_fadvise() for 9.1 (unless someone feels it's ok for 9.0).
>> Currently posix_fadvise() is used right before a log file is closed  
>> so
>> it's effectively not doing anything, when posix_fadvise is to be
>> called.  This patch moves the posix_fadvise() call into 3 other
>> locations within XLogFileInit() where a file handle is returned.  The
>> first case is where an existing open file handle is returned.  The
>> next case is when a file is to be zeroed out.  The third case is
>> returning a file handle, which may be the file that was just zeroed
>> out.
>
> I don't think POSIX_FADV_DONTNEED does what you think it does. It  
> tells the kernel that "you don't need to keep these pages in the  
> cache anymore, I won't be accessing them anymore". If you call it  
> when you open the file, before reading/writing, there is nothing in  
> the cache and the call will do nothing.

Oops, my bad.  I think I was confused by the short description in the  
man page.  I didn't read the longer descriptoon. :( Then would it be  
worth making the this call after the file is zeroed out?

Regards,
Mark