From: Johan Holmberg <johan556@...>
Date: 2011-01-06T04:39:24+09:00
Subject: Re: Getting the actual size of a sparse file

On Wed, Jan 5, 2011 at 5:37 PM, Daniel Berger <djberg96@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.

/Johan Holmberg