linux-kernel - ZAC target (Was: Re: dm-multipath: Accept failed paths for multipath maps)

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Message-ID: <53CD226D.1070309@suse.de>
Date:	Mon, 21 Jul 2014 16:23:41 +0200
From:	Hannes Reinecke <hare@...e.de>
To:	John Utz <John.Utz@....com>, Mike Snitzer <snitzer@...hat.com>
CC:	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kmo@...erainc.com>
Subject: ZAC target (Was: Re: dm-multipath: Accept failed paths for multipath
 maps)

On 07/18/2014 07:04 PM, John Utz wrote:
>> On 07/18/2014 05:31 AM, John Utz wrote:
>>> Thankyou very much for the exhaustive answer! I forwarded on to my
>>> project peers because i don't think any of us where aware of the
>>> existing infrastructure.
>>>
>>> Of course, said infrastructure would have to be taught about ZAC,
>>> but it seems like it would be a nice place to start testing from....
>>>
>> ZAC is a different beast altogether; I've posted an initial set of
>> patches a while back on linux-scsi.
>> But I don't think multipath needs to be changed for that.
>> Other areas of device-mapper most certainly do.
>
> Pretty sure John is working on a new ZAC-oriented DM target.
>
> YUP.
>
> Per Ted T'so's suggestion several months ago, the goal is to create
 > a new DM target that implements the ZAC/ZBC command set and the SMR
 > write pointer architecture so that FSfolksen can try their hand at
 > porting their stuff to it.
>
> It's in the very early stages so there is nothing to show yet, but
 > development is ongoing. There are a few unknowns about how to surface
 > some specific behaviors (new verbs and errors, particularly errors
 > with sense codes that return a write pointer) but i have not gotten
 > far enuf along in development to be able to construct succint and
 > specific questions on the topic so that will have to wait for a bit.
>
I was pondering the 'best' ZAC implementation, too, and found the
'report zones' command _very_ cumbersome to use.
Especially the fact that in theory each zone could have a different 
size _and_ plenty of zones could be present will be making zone 
lookup hellish.

However: it seems to me that we might benefit from a generic
'block boundaries' implementation.
Reasoning here is that several subsystems (RAID, ZAC/ZBC, and things 
like referrals) impose I/O scheduling boundaries which must not be 
crossed when assembling requests.

Seeing that we already have some block limitations I was wondering 
if we couldn't have some set of 'I/O scheduling boundaries' as part
of the request_queue structure.

Kent, Jens; comments here?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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