Hi everyone,

I made more updates to http://pad.ceph.com/p/crush-types after the CDM 
discussion yesterday:

- consolidated notes into a single proposal
- use otherwise illegal character (e.g., ~) as separater for generated 
buckets.  This avoids ambiguity with user-defined buckets.
- class-id $class $id properties for each bucket.  This allows us to 
preserve the derivative bucket ids across a decompile->compile cycle so 
that data does not move (the bucket id is one of many inputs into crush's 
hash during placement).
- simpler rule syntax:

    rule ssd {
            ruleset 1
            step take default class ssd
            step chooseleaf firstn 0 type host
            step emit
    }

My rationale here is that we don't want to make this a separate 'step' 
call since steps map to underlying crush rule step ops, and this is a 
directive only to the compiler.  Making it an optional step argument seems 
like the cleanest way to do that.

Any other comments before we kick this off?

Thanks!
sage


On Mon, 23 Jan 2017, Loic Dachary wrote:

> Hi Wido,
> 
> Updated http://pad.ceph.com/p/crush-types with your proposal for the rule syntax
> 
> Cheers
> 
> On 01/23/2017 03:29 PM, Sage Weil wrote:
> > On Mon, 23 Jan 2017, Wido den Hollander wrote:
> >>> Op 22 januari 2017 om 17:44 schreef Loic Dachary <loic@dachary.org>:
> >>>
> >>>
> >>> Hi Sage,
> >>>
> >>> You proposed an improvement to the crush map to address different device types (SSD, HDD, etc.)[1]. When learning how to create a crush map, I was indeed confused by the tricks required to create SSD only pools. After years of practice it feels more natural :-)
> >>>
> >>> The source of my confusion was mostly because I had to use a hierarchical description to describe something that is not organized hierarchically. "The rack contains hosts that contain devices" is intuitive. "The rack contains hosts that contain ssd that contain devices" is counter intuitive. Changing:
> >>>
> >>>     # devices
> >>>     device 0 osd.0
> >>>     device 1 osd.1
> >>>     device 2 osd.2
> >>>     device 3 osd.3
> >>>
> >>> into:
> >>>
> >>>     # devices
> >>>     device 0 osd.0 ssd
> >>>     device 1 osd.1 ssd
> >>>     device 2 osd.2 hdd
> >>>     device 3 osd.3 hdd
> >>>
> >>> where ssd/hdd is the device class would be much better. However, using the device class like so:
> >>>
> >>>     rule ssd {
> >>>             ruleset 1
> >>>             type replicated
> >>>             min_size 1
> >>>             max_size 10
> >>>             step take default:ssd
> >>>             step chooseleaf firstn 0 type host
> >>>             step emit
> >>>     }
> >>>
> >>> looks arcane. Since the goal is to simplify the description for the first time user, maybe we could have something like:
> >>>
> >>>     rule ssd {
> >>>             ruleset 1
> >>>             type replicated
> >>>             min_size 1
> >>>             max_size 10
> >>>             device class = ssd
> >>
> >> Would that be sane?
> >>
> >> Why not:
> >>
> >> step set-class ssd
> >> step take default
> >> step chooseleaf firstn 0 type host
> >> step emit
> >>
> >> Since it's a 'step' you take, am I right?
> > 
> > Good idea... a step is a cleaner way to extend the syntax!
> > 
> > sage
> > --
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> 
> -- 
> Loïc Dachary, Artisan Logiciel Libre
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