Fwd: Is the fsync() fake on FreeBSD6.1?

I thought folks might be interested in this... note in particular the  
comment about linux.

Begin forwarded message:

> From: Greg 'groggy' Lehey <grog@FreeBSD.org>
> Date: June 26, 2006 11:34:12 PM EDT
> To: leo huang <leo.huang.list@gmail.com>
> Cc: freebsd-performance@freebsd.org
> Subject: Re: Is the fsync() fake on FreeBSD6.1?
>
> On Tuesday, 27 June 2006 at 10:18:47 +0800, leo huang wrote:
>> Hi,
>>
>> I benchmarked MySQL 4.1.18 on FreeBSD 6.1 and Debian 3.1 using  
>> Super Smack
>> 1.3 some days ago.
>>
>> ...
>>
>> The result surprise me. The MySQL Performance on FreeBSD6.1 is about
>> 10 times of on Debian3.1??and the output of iostat also shows it.
>>
>> I know that MySQL uses fsync() to flush both the data and log files
>> at default when using innodb engine(
>> http://dev.mysql.com/doc/refman/4.1/en/innodb-parameters.html). Our
>> evaluating computer only has a 10000RPM SCSI hard disk. I think it
>> can do about 200 sequential fsync() calls per second if the fsync()
>> is real.
>>
>> Is the fsync() on FreeBSD6.1 fake?
>
> My understanding from the last time I looked at the code was that
> fsync does the right thing:
>
>      The fsync() system call causes all modified data and  
> attributes of fd to
>      be moved to a permanent storage device.  This normally results  
> in all in-
>      core modified copies of buffers for the associated file to be  
> written to
>      a disk.
>
> This is not the case for Linux, where fsync syncs the entire file
> system.  That could explain some of the performance difference, but
> not all of it.  I suppose it's worth noting that, in general, people
> report much better performance with MySQL on Linux than on FreeBSD.
>
>> I mean than the data is only written to the drives memory and so can
>> be lost if power goes down.
>
> I don't believe that fsync is required to flush the drive buffers.  It
> would be nice to have a function that did, though.
>
>> And how I can confirm this?
>
> Trial and error?
>
> Greg
> --
> See complete headers for address and phone numbers.

--
Jim Nasby                                    jimn@enterprisedb.com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)