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

Tue Nov 1 17:10:46 UTC 2011

Hi,

In an another example:

[root at bladelinux01 ~]# dmsetup table firsthalf
0 102400 linear 7:0 0

Using the remaining of first half of the same blockdevice /dev/loop0. This
seems to work. So I ma confused here how the mapping works..
[root at bladelinux01 ~]# dmsetup create x --table "0 102400 linear /dev/loop0
102400"
[root at bladelinux01 ~]# dmsetup table x
0 102400 linear 7:0 102400
[root at bladelinux01 ~]# dmsetup table firsthalf
0 102400 linear 7:0 0
[root at bladelinux01 ~]# dmsetup table x
0 102400 linear 7:0 102400
[root at bladelinux01 ~]# dmsetup deps x
1 dependencies  : (7, 0)
[root at bladelinux01 ~]# dmsetup deps firsthalf
1 dependencies  : (7, 0)
[root at bladelinux01 ~]# dmsetup info x
Name:              x
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      253, 7
Number of targets: 1

[root at 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

--sanjana

On Tue, Nov 1, 2011 at 10:22 PM, Sanjana Shari <sanjana.linux at gmail.com>wrote:

> 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 at bladelinux01 ~]# losetup -a
> /dev/loop0: [fd03]:2359319 (/root/dm/dm_test)
> /dev/loop1: [fd03]:2359321 (/root/dm/dm_test1)
> [root at bladelinux01 ~]# blockdev --getsize /dev/loop0
> 204800  < ------ 100 M
> [root at 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 at 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 at bladelinux01 ~]# dmsetup create firsthalf --table "0 102400 linear
> /dev/loop0 0"
> [root at bladelinux01 ~]# dmsetup table firsthalf
>
> 0 102400 linear 7:0 0
> [root at 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 at bladelinux01 ~]# echo 0 102400 linear /dev/loop0 102400 > table
> [root at bladelinux01 ~]# cat table
> 0 102400 linear /dev/loop0 102400
> [root at bladelinux01 ~]# echo 102400 204800 linear /dev/loop1 0 >>table
> [root at bladelinux01 ~]# cat table
> 0 102400 linear /dev/loop0 102400
> 102400 204800 linear /dev/loop1 0
> [root at bladelinux01 ~]# dmsetup create anotherhalf_plus_otherdisk table
> [root at bladelinux01 ~]# dmsetup table anotherhalf_plus_otherdisk
> 0 102400 linear 7:0 102400
> 102400 204800 linear 7:1 0
> [root at 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 at bladelinux01 ~]# blockdev --getsize
> /dev/mapper/anotherhalf_plus_otherdisk
> 307200
>
> [root at 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.
>
>
>
>
> On Tue, Nov 1, 2011 at 2:01 PM, Milan Broz <mbroz at redhat.com> wrote:
>
>> 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
>>
>
>
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