Re: [coLinux-devel] Sparse files?

["Bill C. Riemers" <docbill at freeshell dot org>]

Looks like you are right.  The following sequence works:

  dd if=/dev/zero of=small bs=4096 seek=32 count=0
  cp --sparse=always small sparse
  dd if=/dev/zero bs=64M of=sparse seek=2048 count=0

I seem to be limited on how much I can grow the file each time, but
eventually it can grow up to 64k less than 16 terabytes as a maximum size on
a partition that only has 7 gigabytes of space available.

                                  Bill

----- Original Message ----- 
From: "Daniel Slater" <dan_slater@yahoo.com>
To: "'Bill C. Riemers'" <cygwin@docbill.net>; <bhds@earthlink.net>;
<colinux-devel@lists.sourceforge.net>; <cygwin@cygwin.com>
Sent: Thursday, May 13, 2004 1:20 PM
Subject: RE: [coLinux-devel] Sparse files?


> To Create Sparse files under windows, you must the DeviceIoControl() API
> with the FSCTL_SET_SPARSE flag to create a sparse file. I suspect cygwin's
> dd program is not using this API.
>
> Look on the Wiki site: http://www.colinux.org/wiki/index.php/NiceTools for
> some utilities for creating sparse files.
>
>
> -----Original Message-----
> From: colinux-devel-admin@lists.sourceforge.net
> [mailto:colinux-devel-admin@lists.sourceforge.net] On Behalf Of Bill C.
> Riemers
> Sent: Thursday, May 13, 2004 10:51 AM
> To: bhds@earthlink.net; colinux-devel@lists.sourceforge.net;
> cygwin@cygwin.com
> Subject: [coLinux-devel] Sparse files?
>
> OK.  Then there must be a problem in the way cygwin creates sparse files,
> since the files created by seeking past the end in cygwin always take up
the
> full amount of disk space.  I've verified this with both Windows and
cygwin
> tools.  I'll try taking it up on the cygwin list to see what I can learn.
I
> would like to understand why dd works perfectly for creating sparse files
> under unix, but not under cygwin.
>
> $ df .
> Filesystem           1k-blocks      Used Available Use% Mounted on
> q:                    42756964  17119760  25637204  41% /cygdrive/q
>
> $ dd if=/dev/zero bs=1k of=foo seek=25637204 count=0
> 0+0 records in
> 0+0 records out
>
> $ df .
> Filesystem           1k-blocks      Used Available Use% Mounted on
> q:                    42756964  42756964         0 100% /cygdrive/q
>
> I have figured a workaround for this problem, which is if I have a sparse
> file, I can then successfully truncate it to the desired size with dd.
The
> initial sparse file can be created under Linux where dd works correctly
for
> creating sparse files.
>
> docbill@docbill002 /cygdrive/q
> $ tar xSjf sparse.tar.bz2 huge
>
> docbill@docbill002 /cygdrive/q
> $ dd if=/dev/zero of=huge bs=4M seek=1024 count=0
> 0+0 records in
> 0+0 records out
>
> docbill@docbill002 /cygdrive/q
> $ df .
> Filesystem           1k-blocks      Used Available Use% Mounted on
> q:                    42756964  15777568  26979396  37% /cygdrive/q
>
>                      Bill
>
> ----- Original Message ----- 
> From: "Brian Dunford-Shore" <bhds@earthlink.net>
> To: <colinux-devel@lists.sourceforge.net>
> Sent: Thursday, May 13, 2004 1:02 AM
> Subject: Re: [coLinux-devel] Re: Installer for CoLinux
>
>
> > > Do you really save any filesystem space with NTFS?
> > >
> > > Certainly it is much faster to create a sparse file:
> > >
> > > i.e.
> > > dd if=/dev/zero of=foo bs=4M seek=1024 count=0
> > >
> > > is much faster than
> > >
> > > dd if=/dev/zero of=foo bs=4M count=1024
> > >
> > > but either way Windows will still report the same amount of disk used
> when
> > > you are done.
> > >
> > > As I understand NTFS, it reserves the disk space used for filesystem
> > > holes...  I do not believe VFAT supports file system holes at all.
> > >
> > >                                   Bill
> > >
> >
> > If you do a right-click properties on a sparse file, it shows 'size' and
> 'size
> > on disk'.  The 'size on disk' for a sparse file (just as for compressed
> files)
> > is the real amount of space that Windows uses and reserves.  I have put
> sparse
> > files on partitions where the 'size' is larger than the whole
> partition--Windows
> > is not reserving the disk space.  This is documented also on the
articles
> on the
> > subject on MSDN.
> >
> > You are right about FAT, FAT32/VFAT--they don't support sparse files.  I
> haven't
> > heard if Microsoft's new file system will support sparse files.
> >
> > Brian Dunford-Shore
> > bhds@earthlink.net
> >
> >
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