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



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.

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