On 04/21/2015 09:08 AM, Robert LeBlanc wrote:
> Your logic isn't quite right and from what I understand, this is how it works:
>
> step choose firstn 2 type rack # Choose two racks from the CRUSH map
> (my CRUSH only has two, so select both of them)
> step chooseleaf firstn 2 type host # From the set chosen previously (two
> racks), select a leaf (osd) from from 2 hosts of each rack (each of the set
> returned previously).
>
> If you have size 3, it will pick two OSDs from one rack and one from the
> second (remember that the first rack in placement will sometimes be 'A' and
> sometimes 'B' so the placement won't be totally unbalanced).
OK, that explains why I was thinking you would end up with 4 replicas, 2 in
each rack. For some reason I was superimposing some imaginary step before the
rack, but your first step is root default.
> Where the min_size and max_size comes in could be something like this (this
> is somewhat exaggerated):
>
> Lets say that you want the minimal possible latency and highest bandwidth and
> are OK with losing data (swap partitions or something). You create a pool
> with size 1 and a rule like this:
>
> rule replicated_swap {
> ruleset 0
> type replicated
> min_size 1
> max_size 1
> step take default
> step chooseleaf firstn 0 type host
> step emit
> }
>
> Then you have a pool you want to put on some hosts that have RAID5 prtected
> OSDs, so you don't need as many replications because RAID will protect from
> disk failures:
>
> rule replicated_radi5 {
> ruleset 1
> type replicated
> min_size 2
> max_size 2
> step take raid5
> step chooseleaf firstn 0 type host
> step emit
> }
>
> Then you have a pool that you want "default" protection for 3-4 copies:
>
> rule replicated_default {
> ruleset 2
> type replicated
> min_size 3
> max_size 4
> step take default
> step chooseleaf firstn 0 type host
> step emit
> }
>
> Then you have a pool that you absolutely can't lose data on, so you have lots
> of copies and want it spread throughout the data center:
>
> rule replicated_paranoid {
> ruleset 3
> type replicated
> min_size 5
> max_size 10
> step take default
> step chooseleaf firstn 0 type rack
> step emit
> }
>
> You then specify the rule to use for each pool. Again, the min and max size
> is a selector for the rule. If the actual pool size is outside of the min and
> max, then the rule should not run (I don't know if it actually does this or
> is just a reminder for the human to know what sizes the rule was
> intentionally written for).
Thank your posting these example scenarios. This is definitely helpful.
Regards,
Colin
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