From: Gary Wright <gwtmp01@...>
Date: 2011-01-07T06:11:30+09:00
Subject: Re: Getting the actual size of a sparse file


On Jan 6, 2011, at 3:44 PM, Daniel Berger wrote:
> 
> 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?

I think you'll want to read up on the stat() system call. The POSIX
standard leaves a bit of wiggle room though since while it does specify
that st_blocks must be returned it doesn't specify the size of the blocks.

I'm not sure I understand your concern about 'actual' vs. 'preferred'.
I'm guessing they would be the same in almost any rational implementation
but the main reason for having the information is to perform I/O in
efficiently sized chunks.  In that case, the 'preferred' block size would
seem to be what you want even if the 'actual' block size was different.

Gary Wright