GNU bug report logs - #15578
Parameter -d or --direct to open files with flag O_DIRECT?

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From: Pádraig Brady <P <at> draigBrady.com>
To: Kyle Sallee <kyle.sallee <at> gmail.com>
Cc: 15578 <at> debbugs.gnu.org, Eric Blake <eblake <at> redhat.com>
Subject: bug#15578: Parameter -d or --direct to open files with flag O_DIRECT?
Date: Thu, 10 Oct 2013 09:09:03 +0100

On 10/10/2013 05:49 AM, Kyle Sallee wrote:
> Please forgive the inconvenience.
> Attaching a gzipped patch file
> ensures the integrity of patch file content.
> I should have presented only the idea
> rather than a patch for the implementation.
> 
> I concur dd is already suited to the task.
> 
> Please consider
> 
> | sed 's/^/if=/' | xargs -r --max-lines=1 dd iflag=direct  # in contrast
> with
>                    | xargs -r --max-lines=4096 cat -d --
> 
> Invoking dd 466059 times costs only a slight performance decrease
> as compared with invoking cat 114 times.
> However, this example probably represents rare usage for cat.
> 
> Thanks for granting the time and consideration.

Fair point, but still not worth adding to cat(1)
since it's not special in this regard.

Something like this might be more appropriate:
https://github.com/Feh/nocache

Note that doesn't avoid the page cache completely,
and so may be more performant/portable than O_DIRECT.
(dd has this functionality too as described at 'nocache' at:
 http://www.gnu.org/software/coreutils/manual/html_node/dd-invocation.html)

cheers,
Pádraig.

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