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Re: [dm-devel] [PATCH] updated dm-switch




On Thu, 6 Jun 2013, Jonathan McDowell wrote:

> On Wed, May 22, 2013 at 05:14:18PM -0400, Mikulas Patocka wrote:
> 
> > This is updated dm-switch. It has better documentation and better error
> > reporting in message function.
> ...
> > ---
> > 
> > From: Jim Ramsay <jim_ramsay dell com>
> > 
> > dm-switch is a new target that maps IO to underlying block devices
> > efficiently when there are a large number of fixed-sized address regions
> > but there is no simple pattern to allow for a compact mapping
> > representation such as dm-stripe.
> 
> Interesting. I have been working on something similar recently, for not
> dissimilar reasons. However the approach I had been taking was to extend
> __choose_pgpath to take a pos parameter indicating the block being
> requested, and then add a new dm-pref-hint multipath driver, which
> effectively does a round-robin but prefers any path that has the "pref"
> bit. Currently the dm-pref-hint driver is extremely hacky and reaches
> down into the scsi_device to do an appropriate CDB along each path to
> get the pref-map, or I'd post it for comments.
> 
> I'm interested in why the decision was made to have dm-switch as a separate
> device-mapper layer? Are there a corresponding set of patches to
> multipath-tools which know how to do the setup of each individual path?
> 
> J.

It is a separate target because it was originally developed as such by 
Dell.

It could be possible to turn it into a plugin to dm-mpath but that would 
require changes in the userspace code that is driving it.

Mikulas



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