[dm-devel] Device Mapper Persistent Configuration

[Doug Dumitru]

On Sun, Jun 28, 2015 at 8:21 AM, Gili B <gilib123 at hotmail.com> wrote:

> hi
> We are starting working on a new device mapper module which has targets
> similar
> to the ones in thin module (pool and thin),  but it adds some additional
> features.
> The user should be able to create and use file systems on the thin volumes.
> I wanted to know what is the recommended  way to make the configuration
> with dmsetup
> persistent ?
>

​You should probably use mdraid as an inspiration.  Put a signature block
on every member device.  You can then write a user-space tool to scan
devices and find the collections.  Be sure to use a good "signature" for
your type of device, plus a good UUID signature so that different
generations can be identified.
​


> We want that all file systems will be available after reboot.
>

​If you get the device running early enough, then fstab should be able to
mount file systems.
​


> Should we write our own configuration tool?  should we add special
> configuration files?
>

​You will probably need a user-space tool.  You can probably get away with
no configuration file, but you should again use mdraid as inspiration so
that your user-space tool gets limits of where to look for member pieces.
​


> Should the kernel module we write , automatically discover all the
> configration and initialize accordingly?
>
>
> Regarding debug - Is there a generic way to run dmsetup for example that
> prints some inside details on the targets? (for example number of
> outstanding ios, memory used etc)
>

​You can do this three ways.  1)  build in IOCTL into your module that can
query stuff.  2) export stuff in /sys.  3) export stuff in /proc.  I
personally like exporting debug stuff into /proc.  Remember that printk is
your friend.  I have even created "command" file in proc that then dump
debug information with printk.  If you use /proc, you need to handle
multiple instances of your mapper.  I use
/proc/{my_project_name}/{instance_name}/...
​


>
>
> Thanks
> -gili
>
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>



-- 
Doug Dumitru
EasyCo LLC
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