linux-kernel - Re: How does ext2 implement sparse files?

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Message-ID: <47A2FD9A.8020809@cosmosbay.com>
Date:	Fri, 01 Feb 2008 12:08:10 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Shuduo Sang <sangshuduo@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Lars Noschinski <lklml@...noschinski.de>,
	linux-kernel@...r.kernel.org
Subject: Re: How does ext2 implement sparse files?

Shuduo Sang a écrit :
> On Feb 1, 2008 2:14 AM, Andi Kleen <andi@...stfloor.org> wrote:
>   
>> Lars Noschinski <lklml@...noschinski.de> writes:
>>
>>     
>>> For an university project, we had to write a toy filesystem (ext2-like),
>>> for which I would like to implement sparse file support. For this, I
>>> digged through the ext2 source code; but I could not find the point,
>>> where ext2 detects holes.
>>>
>>> As far as I can see from fs/buffer.c, an hole is a buffer_head which is
>>> not mapped, but uptodate. But I cannot find a relevant source line,
>>> where ext2 makes usage of this information.
>>>       
>> It does not explicitely detect holes; holey data is just never written
>> so no space for it is allocated.
>>
>>     
>
> does anybody know how to make a hole in a large file which already has
> real content from user space application?
> In my project I need this function to delete a piece of content from
> an exist large effectively.
> thanks.
>
>   
Some OSes use fcntl() F_FREESP/F_FREESP64 to be able to free allocated 
space in files (ie make holes if supported by underlying fs)

AFAIK, linux can generically do this only at the end of a file 
(ftruncate()) , not at random points.

XFS has special support for FREESP (it comes from IRIX), implemented as 
an ioctl()

Check for XFS_IOC_FREESP and XFS_IOC_UNRESVSP in fs/xfs/xfs_fs.h





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