[ceph-users] Mimic - EC and crush rules - clarification

Wladimir Mutel mwg at mwg.dp.ua
Thu Nov 1 14:06:04 PDT 2018

David Turner wrote:
> Yes, when creating an EC profile, it automatically creates a CRUSH rule 
> specific for that EC profile.  You are also correct that 2+1 doesn't 
> really have any resiliency built in.  2+2 would allow 1 node to go down 
> while still having your data accessible.  It will use 2x data to raw as 

	Is not EC 2+2 the same as 2x replication (i.e. RAID1) ?
	Is not EC benefit and intention to allow equivalent replication
	factors be chosen between >1 and <2 ?
	That's why it is recommended to have m<k in EC algorithm
	parameters. Because when you have m==k, it is equivalent to 2x
	replication, with m==2k - to 3x replication and so on.
	And correspondingly, with m==1 you have equivalent reliability
	of RAID5, with m==2 - that of RAID6, and you start to have more
	"interesting" reliability factors only when you could allow m>2
	and k>m. Overall, your reliability in Ceph is measured as a
	cluster rebuild/performance degradation time in case of
	up-to m OSDs failure, provided that no more than m OSDs
	(or larger failure domains) have failed at once.
	Sure, EC is beneficial only when you have enough failure domains
	(i.e. hosts). My criterion is that you should have more hosts
	than you have individual OSDs within a single host.
	I.e. at least 8 (and better >8) hosts when you have 8 OSDs
	per host.

> opposed to the 1.5x of 2+1, but it gives you resiliency.  The example in 
> your command of 3+2 is not possible with your setup.  May I ask why you 
> want EC on such a small OSD count?  I'm guessing to not use as much 
> storage on your SSDs, but I would just suggest going with replica with 
> such a small cluster.  If you have a larger node/OSD count, then you can 
> start seeing if EC is right for your use case, but if this is production 
> data... I wouldn't risk it.

> When setting the crush rule, it wants the name of it, ssdrule, not 2.

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