Re: truncate head of file?

["Lukáš Czerner" <lczerner@redhat.com>]

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On Wed, 20 Aug 2014, Howard Chu wrote:

> Date: Wed, 20 Aug 2014 00:00:56 -0700
> From: Howard Chu <hyc@symas.com>
> To: Lukáš Czerner <lczerner@redhat.com>
> Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>
> Subject: Re: truncate head of file?
> 
> Lukáš Czerner wrote:
> > On Tue, 19 Aug 2014, Howard Chu wrote:
> > 
> > > Date: Tue, 19 Aug 2014 22:45:16 -0700
> > > From: Howard Chu <hyc@symas.com>
> > > To: linux-fsdevel <linux-fsdevel@vger.kernel.org>
> > > Subject: truncate head of file?
> > > 
> > > Was thinking it would be very handy to have a truncate() variant that
> > > deletes
> > > pages from the head of a file. This could be leveraged to make logfiles
> > > easier
> > > to maintain, as an example. Anyone else interested, think this would be
> > > nice
> > > to have?
> > > 
> > > (Note - not the same as just punching holes in the beginning of the file -
> > > we
> > > want the beginning of the file to advance as well, past the deleted
> > > pages.)
> > 
> > I am not really sure I understand the behaviour you'd like to see.
> > Can you please explain the behaviour including more concrete use
> > case ?
> 
> For example - we have a logfile (opened O_APPEND) that grows continuously. We
> want to delete some old log info from the head of the file. We could use "hole
> punching" to cause a specific range of data to be freed, but that just leaves
> a sparse file. If we were to cat this file the read() would have to advance
> thru all of that empty space before arriving at actual log data. We want both
> the data to be freed and for the logical beginning of the file to be moved
> forward, to match the location of where the remaining data begins.
> 
> Freeing the space would be simplest if we just deallocate X pages from the
> file, and then the beginning of the file becomes the beginning of the first
> remaining page of the file.

Ok, now I understand. It is exactly what FALLOC_FL_COLLAPSE_RANGE is
for. It has been merged into mainline with commit v3.14-rc1-1-g00f5e61

However it will not work with O_APPEND nor does any other fallocate
flag except pure fallocate, so not even punch hole would have
worked.

-Lukas