Re: [boost] Random Access to Files (RAF) - any interest ?

From: Slava, Alex (raf.devel_at_[hidden])
Date: 2006-05-25 04:35:07

> > More realistic feature request would be support
> > for sparse files.
> >
> > The empty areas could be used for appending
> > new data efficiently, normal operations on RAF
> > should ignore then.
> >
> > Support for sparse files is available in Win32 via
> > DeviceIoControl(FSCTL_QUERY_ALLOCATED_RANGES).
> >
> > I have vague feeling Linux has something like that too.
>
> ZFS on Solaris 10 has an extension to the the Posix lseek() flags, SEEK_DATA
> and SEEK_HOLE.  It makes it easy to find the holes in your files.  I may be
> wrong here but I don't think Linux supports this.
>
Sparce file handling was described in Kernighan book more then 20
years ago. I think that most UNIX-systems produces "holes" by default
(Linux do this on ext2/ext3 and XFS for sure). It is not a problem to
RAF. It just ask a system to allocate space and it is not important
whether the data allocated or "hole" created. when writing in the
"hole" system will allocate blocks for a file. There may be a problem
that there is no more space on device, exactly such case is descried
in MSDN (using memory-maped file on sparce or comprressed file). We
will just check for an error in such cases.

-- 
Svyatoslav Trukhanov,
Oleksii Ursulenko