Documentation on device-mapper and friends

[neha naik]

Hi Greg,
  Thanks for the information. I have another question :).
  Is there some less flexibility if we use device mapper target?
  For example in block device driver you can use the api such that it won't
use the OS io scheduler, so the io comes directly to the block device
driver through the   'make_request' call. With the device mapper i don't
think that happens(looking at the api calls). Does this mean that stuff
like io scheduling. barrier control etc is done
 by the device mapper itself and we can focus only on 'mapping' the io.

Regards,
Neha

On Wed, May 8, 2013 at 8:11 AM, Greg Freemyer <greg.freemyer at gmail.com>wrote:

> 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
> <gauravmahajan2007 at gmail.com> 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 <nehanaik27 at gmail.com> wrote:
> >>
> >> 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
> >> <gauravmahajan2007 at gmail.com> wrote:
> >>>
> >>> 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
> >>> <anatol.pomozov at gmail.com> wrote:
> >>>>
> >>>> Hi
> >>>>
> >>>> On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit at gmail.com>
> >>>> wrote:
> >>>> > On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer
> >>>> > <greg.freemyer at gmail.com> wrote:
> >>>> >> 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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> >>>
> >>>
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