linux-kernel - Re: [dm-devel] [PATCH 11/13] dm: enable request-based dm

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Date:	Fri, 24 Oct 2008 13:22:46 +0530
From:	"Nikanth K" <nikanth@...il.com>
To:	"device-mapper development" <dm-devel@...hat.com>
Cc:	jens.axboe@...cle.com, agk@...hat.com,
	James.Bottomley@...senpartnership.com, j-nomura@...jp.nec.com,
	k-ueda@...jp.nec.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	"Nikanth Karthikesan" <knikanth@...e.de>
Subject: Re: [dm-devel] [PATCH 11/13] dm: enable request-based dm

On Fri, Sep 12, 2008 at 8:16 PM, Kiyoshi Ueda <k-ueda@...jp.nec.com> wrote:
> This patch enables request-based dm.
>
> o Request-based dm and bio-based dm coexist, since there are
>  some target drivers which are more fitting to bio-based dm.
>  Also, there are other bio-based devices in the kernel
>  (e.g. md, loop).
>  Since bio-based device can't receive struct request,
>  there are some limitations on device stacking between
>  bio-based and request-based.
>
>                     type of underlying device
>                   bio-based      requeset-based
>   ----------------------------------------------
>    bio-based         OK                OK
>    request-based     NG                OK
>

So will some configurations would become impossible, if a target is
made request-based and bio-based one is removed. Hope both
types of targets would co-exist and would be selectable for sometime.

>  The device type is recognized by the queue flag in the kernel,
>  so dm follows that.
>
> o The type of a dm device is decided at the first table loading time.
>  Until then, mempool creations are deferred, since mempools for
>  request-based dm are different from those for bio-based dm.
>  Once the type of a dm device is decided, the type can't be changed.
>
> o Currently, request-based dm supports only tables that have a single
>  target.

Barrier support should be straight forward?

Thanks
Nikanth Karthikesan
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