[dm-devel] creating two device mapper files for an underline blockdevice

[Sanjana Shari <sanjana linux gmail com>]

Hello,

I am trying to create two device mapper files for an underline blockdevice. One file( half1) will be mapped to the first half of the block-device. and the second half (half2) will be mapped to the rest of the space in the same block device.

1.Using dd command I have created a 100 MB file.

# dd if=/dev/zero of=dm_test bs=512 count=204800

2. associated it with loop device:
# losetup /dev/loop0 /tmp/dm_test

# blockdev --getsize /dev/loop0
204800

using dmsetup to create dm file.
# dmsetup create half --table "0 102400 linear /dev/loop0 0"

[root bladelinux01 root]# dmsetup table half
0 102400 linear 7:0 0
[root bladelinux01 root]# dmsetup info half
Name:              half
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      253, 6
Number of targets: 1


[root bladelinux01 root]# ls -l /dev/mapper/half
lrwxrwxrwx. 1 root root 7 Nov  1 01:44 /dev/mapper/half -> ../dm-6

But if I use the other part of the disk to map. The command is not successful

[root bladelinux01 root]#  dmsetup create half1 --table "102401 102399 linear /dev/loop0 102400"
device-mapper: reload ioctl failed: Invalid argument
Command failed

[root bladelinux01 root]#  dmsetup create half1 --table "102401 102399 linear /dev/loop0 0"
device-mapper: reload ioctl failed: Invalid argument
Command failed

The format used seems to be correct according to the man page of dmsetup. Please guide me. I am not sure what am I missing.

-- sanjana