[dm-devel] device mapper mapping across reboot

[Jitendra]

>lvm2 is exactly solving this problem as it maintains consistent
>'metadata' on every device - so upon reboot devices are discovered and
>from their metadata dm tables are actived/restored.

Got it.

>So  are you looking for recreation of all the lvm2 infrastructure for
>this relatively quite complex task ?
Not exactly, but I want to create mapper device created before the root and
other fs get mounted so that I can track I/Os in target.

>Or you just want to 'create' DM after kernel is booted ?
dmsetup create with my target.

>Or you even want to pass 'DM' table line on kernel boot option line -
>so even your boot device is a 'DM' device ?

It is something exactly, I wanna to do.

eg. I wrote a basic target as explained here
http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html

Now, to use this target (kernel module), I need to create mapper device as
echo 0 <size_of_device> basic_target /Path/to/your/device 0 | dmsetup create my_basic_dm_device

After creation of device as /dev/mapper/my_basic_dm_device for /dev/sda, if I do I/O from
/dev/mapper/my_basic_dm_device, then all I/O goes through basic_target before it
hits to /dev/sda.

Now, if system is booted and disk is offline then it is very easy to create
mapper device. But now let suppose I want to boot on
/dev/mapper/my_basic_dm_device instead of /dev/sda1 etc. then I have to create
mapper device even before it get switch root.

This is same case for other disks as well. So there should be a way so that I
can use device mapper framework after reboot.

---
Jitendra