linux-kernel - Re: [ext4] Documentation patch

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Message-ID: <493B0BAD.10004@redhat.com>
Date:	Sat, 06 Dec 2008 17:33:01 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Theodore Tso <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Harald Arnesen <skogtun.harald@...il.com>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [ext4] Documentation patch

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@....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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