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

David Turner drakonstein at gmail.com
Fri Nov 16 12:27:37 PST 2018

The difference for 2+2 vs 2x replication isn't in the amount of space being
used or saved, but in the amount of OSDs you can safely lose without any
data loss or outages.  2x replication is generally considered very unsafe
for data integrity, but 2+2 would is as resilient as 3x replication while
only using as much space as 2x replication.

On Thu, Nov 1, 2018 at 11:25 PM Wladimir Mutel <mwg at mwg.dp.ua> wrote:

> 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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