Why does journal mode outperforms all other modes when reading and writing data at the same time?

Why does journal mode outperforms all other modes when reading and writing data at the same time?

[jingguo yao]

Documentation/filesystems/ext4.txt has the following sentence:

  This mode is the slowest except when data needs to be read from and
  written to disk at the same time where it outperforms all others
  modes.

And the following link talks about it in more details. Can anybody give
the reason?

http://www.ibm.com/developerworks/library/l-fs8/index.html#4

--
Jingguo

Re: Why does journal mode outperforms all other modes when reading and writing data at the same time?

[Jan Kara]

On Tue 28-05-13 22:39:18, jingguo yao wrote:
> Documentation/filesystems/ext4.txt has the following sentence:
> 
>   This mode is the slowest except when data needs to be read from and
>   written to disk at the same time where it outperforms all others
>   modes.
> 
> And the following link talks about it in more details. Can anybody give
> the reason?
> 
> http://www.ibm.com/developerworks/library/l-fs8/index.html#4
  Not sure why exactly data=journal was faster in that workload. But
generally data=journal is known to help if you have shortlived files which
are then written only to the journal and deleted before they are copied to
the final location on disk. Also it may help by penalizing the writer so
that readers have better chance to do their work...

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR