Re: How does ext2 implement sparse files?

[Eric Dumazet]

Shuduo Sang a Ãcrit :
> On Feb 1, 2008 2:14 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>   
> > Lars Noschinski <lklml@xxxxxxxxxxxxxxxx> 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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