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

[Milan Broz]

On 11/01/2011 07:08 AM, Sanjana Shari wrote:

> But if I use the other part of the disk to map. The command is not successful
> 
> [root at bladelinux01 root]#  dmsetup create half1 --table "102401 102399 linear /dev/loop0 102400"
> device-mapper: reload ioctl failed: Invalid argument
> Command failed

You have to always map the whole table (starting at sector 0).
If you see syslog, you can see why
  device-mapper: table: 254:2: linear: Gap in table
  device-mapper: ioctl: error adding target to table

Anyway, for the info how it works, example how to switch active table:

1. create linear device mapped to /dev/sdb

  # dmsetup create x --table "0 10000 linear /dev/sdb 0"
  # dmsetup table x
  0 10000 linear 8:16 0

2. Remap the first half of device to another target, here "error"

 - load new (yet inactive) table
 # echo -e "0 5000 error\n5000 10000 linear /dev/sdb 5000" | dmsetup load x                                      

 - so now you have one active table and one inactive (prepared for switch)
 # dmsetup table x
 0 10000 linear 8:16 0

 # dmsetup table x --inactive
 0 5000 error 
 5000 10000 linear 8:16 5000

3. switch to new (inactive) table (note that "dmsetup suspend" here is implicit)
  # dmsetup resume x

  # dmsetup table x
  0 5000 error 
  5000 10000 linear 8:16 5000


If you want more info about this low level DM operation, read
http://people.redhat.com/agk/talks/FOSDEM_2005/
http://mbroz.fedorapeople.org/talks/DeviceMapperBasics/
(old, but still useful, I hope :-)

Milan