From: Daniel Berger <djberg96@...>
Date: 2011-01-07T05:44:18+09:00
Subject: Re: Getting the actual size of a sparse file



On Jan 5, 12:39 pm, Johan Holmberg <johan...@gmail.com> wrote:
> On Wed, Jan 5, 2011 at 5:37 PM, Daniel Berger <djber...@gmail.com> wrote:
> > Hi,
>
> > How do you get the true size of a sparse file? Using /var/log/lastlog
> > on Ubuntu as an example I see this with "ls -lh"
>
> > 287K lastlog
>
> > With "ls -sh" I see this:
>
> > 40K lastlog
>
> > A File.stat call reveals this:
>
> > #<File::Stat
> >  dev=0x801,
> >  ino=5249695,
> >  mode=0100664 (file rw-rw-r--),
> >  nlink=1,
> >  uid=0 (root),
> >  gid=43 (utmp),
> >  rdev=0x0 (0, 0),
> >  size=292876,
> >  blksize=4096,
> >  blocks=80,
> >  atime=Mon Jan 03 16:03:24 -0700 2011 (1294095804),
> >  mtime=Thu Oct 21 11:34:51 -0600 2010 (1287682491),
> >  ctime=Thu Oct 21 11:34:51 -0600 2010 (1287682491)>
>
> > Multiplying blocks * blksize doesn't seem to match up, either.
>
> See stat(2):
>
>        The st_blocks field indicates the number of  blocks  allocated  to  the
>        file,  512-byte  units.  (This may be smaller than st_size/512 when the
>        file has holes.)
>
>        The st_blksize field gives the "preferred" blocksize for efficient file
>        system  I/O.  (Writing to a file in smaller chunks may cause an ineffi-
>        cient read-modify-rewrite.)
>
> So "blksize" has nothing to do with the size of the "blocks". They are
> always counted in 512-byte units.

Oh, wow, I don't think I knew that. It's strikes me as particularly
bizarre that they would return some notion of a "preferred block size"
instead of the actual block size. Seriously, what's the use of that?

Now I need to check other platforms (Solaris, HP-UX) to see if they
use 512 byte convention.

Is this something that's universal? Or is it something I can get via a
C call somewhere?

Regards,

Dan