linux-kernel - Re: [dm-devel] [PATCH 1/1] block: rework flush sequencing for blk-mq

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Date:	Mon, 17 Mar 2014 11:55:18 +0000
From:	"Bryn M. Reeves" <bmr@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
CC:	Mike Snitzer <snitzer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [dm-devel] [PATCH 1/1] block: rework flush sequencing for blk-mq

On 03/14/2014 02:13 PM, Mike Snitzer wrote:
> Yeah, not sure why single path scsi_debug "just works", maybe it is a
> "feature" of the older multipathd I have kicking around?, but for basic
> data path testing scsi_debug is a quick means to an end.  I can look
> closer at _why_ it gets multipathd in a bit.  But maybe Ben or Hannes
> will have quicker insight?

Do you have find_multipaths set in multipath.conf? It defaults to off.
If it's enabled then multipath will only create maps for WWIDs it
already knows about (listed in /etc/multipath/wwids) unless there are at
least two devices with the same WWID or the user forces the operation:

Mar 17 12:26:50 | checking if sdm should be multipathed
Mar 17 12:26:50 | wwid 35333333000000000 not in wwids file, skipping sdm

Turn it off and multipath will happily map a single path scsi_debug for me:

# multipath
create: mpathg (35333333000000000) undef Linux,scsi_debug
size=1.0G features='0' hwhandler='0' wp=undef
`-+- policy='round-robin 0' prio=1 status=undef
  `- 8:0:0:0 sdm 8:192 undef ready  running

Iirc this is enabled on at least some current distros by default to
avoid creating maps for USB keys and the like.

Regards,
Bryn.

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