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
step take default
step chooseleaf firstn 0 type host
step emit
}
What do you think ?
Cheers
[1] http://pad.ceph.com/p/crush-types http://tracker.ceph.com/projects/ceph/wiki/CDM_01-FEB-2017
--
Loïc Dachary, Artisan Logiciel Libre
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