The block layers can be layered both ways.  DM is the newer
infrastructure and was created in the early days of 2.6

If what I was writing could fit into a dm-target, that is what I would do.

There are significant projects like drbd and mdraid that are not
dm-targets, but I think their is a long term goal to incorporate
mdraid's functionality at a minimum into dm.  I doubt drbd is ever
moved to dm.  It is just too big of a project and in use in lots of
production server environments.

Greg

On Tue, May 7, 2013 at 1:46 AM, Gaurav Mahajan
[off-list ref] wrote:

Hi  Neha,

LVM uses device mapper. Advantages of using device mapper is that you can
stack different dm-targets on each other.
I am really not aware of block device drivers.

May be Greg can help us understand the actual pros and cons.

Thanks,
Gaurav


On Wed, May 1, 2013 at 9:45 PM, neha naik [off-list ref] wrote:

quoted

Hi Gaurav,
 I went through your blog and it is really informative. But after reading
that i realized that i have a question:
  If I want to write a block device driver which is going to sit on lvm
(and do some functionality on top of it) then should i go for the block
device driver api
  or write it as a device mapper target. What are the
advantages/disadvantages of both the approaches.

Regards,
Neha


On Tue, Apr 30, 2013 at 4:24 AM, Gaurav Mahajan
[off-list ref] wrote:

quoted

Hi Amit,

I had compiled some notes on my blog.
Here are some links on writing your own device mapper target.
http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html

Concept of device mapper target.
http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html

Thanks,
Gaurav.


On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov
[off-list ref] wrote:

quoted

Hi

On Mon, Apr 29, 2013 at 9:51 AM, amit mehta [off-list ref]
wrote:

quoted

On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer
[off-list ref] wrote:

quoted

A nice diagram of the overall storage subsystem is at
http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html

Dm is just a single block in it, but it can help to see where it fits
in overall.

Btw: that diagram doesn't show the legacy ata driver that creates
/dev/hdx style devices.  Has that been dropped while I wasn't paying
attention?  I haven't used it in years, but I thought it was still used on
embedded systems.

Thank you for sharing the link, but I'm looking for more
detailed information on I/O stack in Linux, dm-mapper and
multipath in particular.

Some docs about multipath can be found here

http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide
http://christophe.varoqui.free.fr/refbook.html

The userspace part for tools is here
http://sourceware.org/lvm2/

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