From: Brian Candler <b.candler@...>
Date: 2010-10-09T21:35:32+09:00
Subject: Re: Getting the size of a block device

Terry Michaels wrote:
> Obviously, I could wrap a system call around a Linux utility or read a
> proc file... but I am curious if there is a 'built-in' Ruby method.

The ioctl is called BLKGETSIZE/BLKGETSIZE64, and a quick grep of the 
ruby source code doesn't find it.

Maybe someone else has published an extension which wraps the API 
directly, but you could just use IO.ioctl directly.

$ sudo blockdev --getsz /dev/sda
976773168

$ sudo strace blockdev --getsz /dev/sda 2>&1 | grep ioctl
ioctl(3, BLKGETSIZE64, 0x7fff486af378)  = 0

$ sudo strace -e raw=ioctl blockdev --getsz /dev/sda 2>&1 | grep ioctl
ioctl(0x3, 0x80081272, 0x7fffacd73d68)  = 0

$ sudo ruby -e 'v = [0].pack("Q"); 
File.open("/dev/sda").ioctl(0x80081272, v); v = v.unpack("Q")[0]; puts 
v'
500107862016

But I'm not sure that's exactly right; it gives the size in bytes rather 
than blocks.
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