Re: fsync() Performance Issue

[Hans Reiser <reiser@namesys.com>]

Chris Mason wrote:

>On Sat, 2002-05-04 at 10:59, Hans Reiser wrote:
>  
>
>>So how about if you revise fsync so that it always sends data blocks to 
>>the journal not to the main disk?
>>    
>>
>
>This gets a little sticky.
>
>Once you log a block, it might be replayed after a crash.  So, you have
>to protect against corner cases like this:
>
>write(file)
>fsync(file) ; /* logs modified data blocks */
>write(file) ; /* write the same blocks without fsync */
>sync ;        /* use expects new version of the blocks on disk */
><crash>
>
>During replay, the logged data blocks overwrite the blocks sent to disk
>via sync().
>
>This isn't hard to correct for, every time a buffer is marked dirty, you
>check the journal hash tables to see if it is replayable, and if so you
>log it instead (the 2.2.x code did this due to tails).  This translates
>to increased CPU usage for every write.
>
Significant increased CPU usage?

>
>I'd rather not put it back in because it adds yet another corner case to
>maintain for all time.  Most of the fsync/O_SYNC bound applications are
>just given their own partition anyway, so most users that need data
>logging need it for every write.
>
most users don't know enough to turn it on....;-)

>
>-chris
>
>
>
>
>
>
>  
>