Re: truncate head of file?

[Howard Chu <hyc@symas.com>]

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.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/
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