device-mapper linear documentation parameters

Question

Here is the documentation for using device-mapper linear: https://www.kernel.org/doc/Documentation/device-mapper/linear.txt

I am trying to understand this documentation. It says it takes 2 parameters: dev path and offset. Yet all the examples I find show other stuff both in front of and after linear. You can see examples in the documentation and here: http://pig.made-it.com/devicemapper.html

Here is an example from that last link:

echo 0 $(blockdev --getsize /dev/loop1) linear /dev/loop1 0 > /tmp/onedisk SIZE1=$(blockdev --getsize /dev/loop1) SIZE2=$(blockdev --getsize /dev/loop2) echo ${SIZE1} ${SIZE2} linear /dev/loop2 0 >> /tmp/onedisk dmsetup create twoasone /tmp/onedisk

As far as I can tell, the first line is taking the output of some stuff and putting it into /tmp/onedisk. Why is there stuff in front of linear, and what does it do?

Ok, I have been looking at this a for a while now. My guess is that this is basically three parameters to echo.

1. 0
2. $(blockdev --getsize /dev/loop1)
3. linear /dev/loop1 0

The output of echo is then stored in /tmp/onedisk. The contends of tmp/ondisk eventually become a parameter for dmsetup create. Is that correct, and am I correct in thinking that really I need to be looking at the documentation for dmsetup create if I want to understand why a 0 and size are being stored at the beginning of /tmp/onedisk?

Answer 1

In the first line from the above example, linear is never actually run. This is all just being echoed out into /tmp/onedisk. I know this because the output for tmp/onedisk is very similar to

0 2048 linear /dev/loop0 0

linear does not get used until the last line. The last line runs dmsetup with the create command. twoasone is the device name passed to the create command. /tmp/onedisk is also passed to the create command as containing the table format. So linear is actually a table format option. You can see more information about it from the dmsetup manual by running man dmsetup. Look for "linear" under the "TABLE FORMAT" section.