[ext4] Documentation patch

This is what I have added to the ext4 patch queue.

							- Ted

Update Documentation/filesystems/ext4.txt

Fix paragraph with recommendations on how to tune ext4 for benchmarks.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Theodore Tso wrote:
> This is what I have added to the ext4 patch queue.
> 
> 							- Ted
> 
> Update Documentation/filesystems/ext4.txt
> 
> Fix paragraph with recommendations on how to tune ext4 for benchmarks.
> 
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
> index 845e691..19bb93f 100644
> --- a/Documentation/filesystems/ext4.txt
> +++ b/Documentation/filesystems/ext4.txt
> @@ -58,13 +58,18 @@ Note: More extensive information for getting started with ext4 can be
>  
>  	# mount -t ext4 /dev/hda1 /wherever
>  
> -  - When comparing performance with other filesystems, remember that
> -    ext3/4 by default offers higher data integrity guarantees than most.
> -    So when comparing with a metadata-only journalling filesystem, such
> -    as ext3, use `mount -o data=writeback'.  And you might as well use
> -    `mount -o nobh' too along with it.  Making the journal larger than
> -    the mke2fs default often helps performance with metadata-intensive
> -    workloads.
> +  - When comparing performance with other filesystems, it's always
> +    important to try multiple workloads; very often a subtle change in a
> +    workload parameter can completely change the ranking of which
> +    filesystems do well compared to others.  When comparing versus ext3,
> +    note that ext4 enables write barriers by default, while ext3 does
> +    not enable write barriers by default.  So it is useful to use
> +    explicitly specify whether barriers are enabled or not when via the
> +    '-o barriers=[0|1]' mount option.  

That sentence doesn't quite parse...

> + When tuning ext3 for best
> +    benchmark numbers, it is often worthwhile to try changing the data
> +    journaling mode; '-o data=writeback,nobh' can be faster for some
> +    workloads.  

It should probably be made obvious that this has a security implication
(stale data exposed, right?)

-Eric

> + A large journal can also be helpful for
> +    metadata-intensive workloads.
>  
>  2. Features
>  ===========

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Good points.  OK, how about this?
     	      
  - When comparing performance with other filesystems, it's always
    important to try multiple workloads; very often a subtle change in a
    workload parameter can completely change the ranking of which
    filesystems do well compared to others.  When comparing versus ext3,
    note that ext4 enables write barriers by default, while ext3 does
    not enable write barriers by default.  So it is useful to use
    explicitly specify whether barriers are enabled or not when via the
    '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
    for a fair comparison.  When tuning ext3 for best benchmark numbers,
    it is often worthwhile to try changing the data journaling mode; '-o
    data=writeback,nobh' can be faster for some workloads.  (Note
    however that running mounting with data=writeback can potentially
    leave stale data exposed in recently written files in case of an
    unclean shutdown, which could be a security exposure in some
    situations.)  Configuring the filesystem with a large journal can
    also be helpful for metadata-intensive workloads.

							- Ted
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Theodore Tso wrote:
> Good points.  OK, how about this?
>      	      
>   - When comparing performance with other filesystems, it's always
>     important to try multiple workloads; very often a subtle change in a
>     workload parameter can completely change the ranking of which
>     filesystems do well compared to others.  When comparing versus ext3,
>     note that ext4 enables write barriers by default, while ext3 does
>     not enable write barriers by default.  So it is useful to use
>     explicitly specify whether barriers are enabled or not when via the
>     '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
>     for a fair comparison.  When tuning ext3 for best benchmark numbers,
>     it is often worthwhile to try changing the data journaling mode; '-o
>     data=writeback,nobh' can be faster for some workloads.  (Note
>     however that running mounting with data=writeback can potentially

I'd say "running mounted with data=writeback...."

other than that it looks good to me :)

(sorta nitpicky but it probably won't be touched again for 5 years so
may as well get it right now) :)

-Eric

>     leave stale data exposed in recently written files in case of an
>     unclean shutdown, which could be a security exposure in some
>     situations.)  Configuring the filesystem with a large journal can
>     also be helpful for metadata-intensive workloads.
> 
> 							- Ted

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