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

[Mike Snitzer]

On 8/28/09, Mike Snitzer <snitzer at redhat.com> wrote:
> On Fri, Aug 28 2009 at  1:00am -0400,
> Kiyoshi Ueda <k-ueda at ct.jp.nec.com> wrote:
>
>> 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.
>
> As a stop-gap I'd imagine Linux's 'scsi_debug' could be used for testing
> and debugging purposes.

I looked closer at scsi_debug and it doesn't have the ability to
establish a shared WWID for N SCSI targets.  That aside, it still
wouldn't offer anywhere near as much utility as supporting the various
bio-based DM targets for mpath testing purposes.