Hi Paul, Contrary to what the documentation states at
http://ceph.com/docs/master/rados/troubleshooting/troubleshooting-pg/#crush-gives-up-too-soon the crush ruleset can be modified (an update at https://github.com/ceph/ceph/pull/4306 will fix that). Placement groups will move around, but that's to be expected. Cheers On 06/04/2015 20:40, Paul Evans wrote: > Thanks for the insights, Greg. It would be great if the CRUSH rule for an EC > pool can be dynamically changed…but if that’s not the case, the > troubleshooting doc also offers up the idea of adding more OSDs, and we have > another 8 OSDs (one from each node) we can move into the default root. > However: just to clarify the point of adding OSDs: the current EC profile has > a failure domain of ‘host’... will adding more OSDs still improve the odds of > CRUSH finding a good mapping within the given timeout period? > > BTW, I’m a little concerned about moving all 8 OSDs at once, as we’re skinny > on RAM and the EC pools seem to like more RAM that replicated pools do. > Considering the RAM issue, is adding 2-4 OSDs at a time the recommendation? > (other than adding more RAM). > > -- > *Paul Evans > * > * > * >> This looks like it's just the standard risk of using a pseudo-random >> algorithm: you need to "randomly" map 8 pieces into 8 slots. Sometimes >> the CRUSH calculation will return the same 7 slots so many times in a >> row that it simply fails to get all 8 of them inside of the time >> bounds that are currently set. >> >> If you look through the list archives we've discussed this a few >> times, especially Loïc in the context of erasure coding. See >> http://ceph.com/docs/master/rados/troubleshooting/troubleshooting-pg/#crush-gives-up-too-soon >> for the fix. >> But I think that doc is wrong and you can change the CRUSH rule in use >> without creating a new pool — right, Loïc? >> -Greg > -- Loïc Dachary, Artisan Logiciel Libre
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