Hi Milan, Thanks for the documentation links... it was useful.. one of the link I had gone through it earlier. Couple of questions more which I would post here... Mapping the second half of the first device + the another device to create a new map. here I ma not mapping the whole of first disk. Or I ma not mapping the whole table, but still device creation is sucessfull. [root@bladelinux01 ~]# losetup -a /dev/loop0: [fd03]:2359319 (/root/dm/dm_test) /dev/loop1: [fd03]:2359321 (/root/dm/dm_test1) [root@bladelinux01 ~]# blockdev --getsize /dev/loop0 204800 < ------ 100 M [root@bladelinux01 ~]# blockdev --getsize /dev/loop1 204800 < ------ 100 M Using first half of the block device /dev/loop0. Here I have mapped the table from 0 to 102400 sectors. [root@bladelinux01 ~]# fdisk -lus /dev/loop0 Disk /dev/loop0: 104 MB, 104857600 bytes 255 heads, 63 sectors/track, 12 cylinders, total 204800 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Command is successful.... as expected. [root@bladelinux01 ~]# dmsetup create firsthalf --table "0 102400 linear /dev/loop0 0" [root@bladelinux01 ~]# dmsetup table firsthalf 0 102400 linear 7:0 0 [root@bladelinux01 ~]# dmsetup info firsthalf Name: firsthalf State: ACTIVE Read Ahead: 256 Tables present: LIVE Open count: 0 Event number: 0 Major, minor: 253, 6 Number of targets: 1 Now here I am creating a new device named "anotherhalf_plus_otherdisk" by mapping the remaining first half of the first disk and then the whole of a second block device /dev/loop1. Creating table to use with dmsetup. [root@bladelinux01 ~]# echo 0 102400 linear /dev/loop0 102400 > table [root@bladelinux01 ~]# cat table 0 102400 linear /dev/loop0 102400 [root@bladelinux01 ~]# echo 102400 204800 linear /dev/loop1 0 >>table [root@bladelinux01 ~]# cat table 0 102400 linear /dev/loop0 102400 102400 204800 linear /dev/loop1 0 [root@bladelinux01 ~]# dmsetup create anotherhalf_plus_otherdisk table [root@bladelinux01 ~]# dmsetup table anotherhalf_plus_otherdisk 0 102400 linear 7:0 102400 102400 204800 linear 7:1 0 [root@bladelinux01 ~]# dmsetup info anotherhalf_plus_otherdisk Name: anotherhalf_plus_otherdisk State: ACTIVE Read Ahead: 256 Tables present: LIVE Open count: 0 Event number: 0 Major, minor: 253, 7 Number of targets: 2 device created with size as expected. Agian here I haven't used the start sector as "0" for the second block device, I have used "102400"th sector to start from, which I believe should be continuation of the previous logical sector. < --- Please this part. [root@bladelinux01 ~]# blockdev --getsize /dev/mapper/anotherhalf_plus_otherdisk 307200 [root@bladelinux01 ~]# fdisk -lus /dev/mapper/anotherhalf_plus_otherdisk Disk /dev/mapper/anotherhalf_plus_otherdisk: 157 MB, 157286400 bytes 255 heads, 63 sectors/track, 19 cylinders, total 307200 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Here I have some doubts which needs to be cleared to get a thorough understanding. My questions: The table format to be used is as follows from man page of dmsetup. Each line of the table specifies a single target and is of the form: logical_start_sector num_sectors target_type target_args Here: "logical_start_sector" start sector should be "0" for the start of any block device, rather I would say for the start of any new map would e more appropriate. What do you say...? "num_sectors" as name suggestes would be the number of sectors, which would contribute to the size of the end mapped device. For target type "linear" we use arguments as the following: destination_device start_sector < -------- here which is the start sector..? Is it the logical start scetor of the blockdevice or the physical sector of the blockdevice...? Or how do we define it...? This part is not yet clear to me. <div class="gmail_quote">On Tue, Nov 1, 2011 at 2:01 PM, Milan Broz <<a href="mailto:mbroz@redhat.com">mbroz@redhat.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> <div class="im">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@bladelinux01 root]# dmsetup create half1 --table "102401 102399 linear /dev/loop0 102400" > device-mapper: reload ioctl failed: Invalid argument > Command failed </div>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 <a href="http://people.redhat.com/agk/talks/FOSDEM_2005/" target="_blank">http://people.redhat.com/agk/talks/FOSDEM_2005/</a> <a href="http://mbroz.fedorapeople.org/talks/DeviceMapperBasics/" target="_blank">http://mbroz.fedorapeople.org/talks/DeviceMapperBasics/</a> (old, but still useful, I hope :-) Milan </blockquote></div>