linux-ext4 - Re: [PATCH RFC] add FIEMAP_FLAG

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Date:	Mon, 4 Nov 2013 17:35:30 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dave Chinner <david@...morbit.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	"linux-fsdevel@...r.kernel.org Devel" <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH RFC] add FIEMAP_FLAG_DISCARD support

On Nov 4, 2013, at 2:57 PM, Dave Chinner <david@...morbit.com> wrote:
> On Sun, Nov 03, 2013 at 06:42:00PM -0500, Theodore Ts'o wrote:
>> On Mon, Nov 04, 2013 at 10:14:42AM +1100, Dave Chinner wrote:
>>> 
>>> FIEMAP is not the correct interface for data modifying operations.
>>> It is an interface that returns information about file metadata (i.e
>>> the layout of a file) - it is not an interface for modifying the
>>> contents of the file.
>> 
>> Well, it's been argued that FIEMAP was designed to be extensible, and
>> we should use it for other operations.  Where the bounds of that
>> stretches to is certainly an arguable point, and I can understand your
>> observation that something which causes a change to the contents of
>> the file might not be best choice.
> 
> Yes, I agree it is extensible, but it was never intended to be
> extended into a data modification ioctl. It's for querying metadata
> related to file data, not for modifying file data.

I’d agree with Dave here - there is nothing about FIEMAP that indicates
it is an interface for data modification, only for reporting out the
underlying layout of a file.  I think falloc() is already used in this
regard for hole punching, and it makes more sense to me to extend that
for the discard interface as well.

Cheers, Andreas





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