Re: [PATCH 5/5] dm-mpath: convert to request-based

[Kiyoshi Ueda <k-ueda@ct.jp.nec.com>]

Hi Alasdair,

On 08/28/2009 02:54 AM +0900, Alasdair G Kergon wrote:
> On Tue, Jun 02, 2009 at 04:03:25PM +0900, Kiyoshi Ueda wrote:
>> This patch converts dm-multipath target to request-based from bio-based.
>  
> How much effort would it be to retain the old mpath implementation
> in parallel?
> 
> I'm rather concerned that we're losing some useful functionality in
> 2.6.31 with this patch - stacking over bio-based devices (test beds use
> this and it's helpful for debugging), barrier support - and supporting
> both would make it easier for people to compare the two implementations
> and stick to the old one if in their particular circumstances it worked
> better.
> 
> Perhaps, dm-mpath could just register two targets (like snapshot does),
> one bio-based, and one rq-based, sharing most of the functions with
> wrappers to indicate which is which where necessary?

Such wrappers need to be made very well to share codes as much as
possible.  Otherwise, we won't be able to maintain the non-default
(bio-based?) code, then people won't be able to use it even for
testing/debugging.
Also we need to consider the user interface so that user-space tools
won't be confused.

I'll look into this when I finish the barrier implementation of
request-based dm. (hopefully 2.6.32 timeframe, maybe 2.6.33)

By the way, only for testing/debugging purposes, making request-based
error/linear targets (e.g. named like error_rq/linear_rq) may be an
alternative.

What do you think?

Thanks,
Kiyoshi Ueda