Hi, 

You need to set the min_size to 2 in crush rule.  

The exact location and replication flow when a client writes data
depends on the object name and num of pgs. the crush rule determines
which osds will serve a pg, the first is the primary osd for that pg.
The client computes the pg from the object name and writes the object to
the primary osd for that pg, then primary osd is then responsible to
replicate with the other osds serving this pg. So for the same client,
some objects will be sent to datacenter 1 and some to 2 and the osds
will do the rest. 

The other point is regarding how to setup monitors across 2 datacenters
and be able to function if one goes down, this is tricky since monitors
do require an odd number and form a quorum. This link my is quite
interesting, i am not sure if there are better ways to do it: 

https://www.sebastien-han.fr/blog/2013/01/28/ceph-geo-replication-sort-of/


Maged 

On 2018-02-14 04:12, ST Wong (ITSC) wrote:

> Hi,
> 
> Thanks for your advice,
> 
> -----Original Message-----
> From: ceph-users [mailto:ceph-users-boun...@lists.ceph.com] On Behalf Of Luis 
> Periquito
> Sent: Friday, February 09, 2018 11:34 PM
> To: Kai Wagner
> Cc: Ceph Users
> Subject: Re: [ceph-users] Newbie question: stretch ceph cluster
> 
> On Fri, Feb 9, 2018 at 2:59 PM, Kai Wagner <kwag...@suse.com> wrote: Hi and 
> welcome,
> 
> On 09.02.2018 15:46, ST Wong (ITSC) wrote:
> 
> Hi, I'm new to CEPH and got a task to setup CEPH with kind of DR feature.
> We've 2 10Gb connected data centers in the same campus.    I wonder if it's
> possible to setup a CEPH cluster with following components in each 
> data
> center:
> 
> 3 x mon + mds + mgr In this scenario you wouldn't be any better, as loosing a 
> room means loosing half of your cluster. Can you run the MON somewhere else 
> that would be able to continue if you loose one of the rooms?

Will it be okay to have 3 x MON per DC so that we still have 3 x MON in
case of losing 1 DC ?  Or need more in case of double fault - losing 1
DC and failure of any MON in remaining DC will make the cluster stop
working?

>> As for MGR and MDS they're (recommended) active/passive; so one per room 
>> would be enough.
> 
> 3 x OSD (replicated factor=2, between data center)

>> replicated with size=2 is a bad idea. You can have size=4 and
>> min_size=2 and have a crush map with rules something like:

rule crosssite {
        id 0
        type replicated
        min_size 4
        max_size 4
        step take default
        step choose firstn 2 type room
        step chooseleaf firstn 2 type host
        step emit
}

>> this will store 4 copies, 2 in different hosts and 2 different rooms.

Does it mean for new data write to hostA:roomA, replication will take
place as following?
1. from hostA:roomA to hostB:roomA
2. from hostA:roomA to hostA, roomB 
3. from hostB:roomA to hostB, roomB 

If it works in this way, can copy in 3 be skipped so that for each piece
of data, there are 3 replicas - original one, replica in same room, and
replica in other room, in order to save some space?

Besides, would also like to ask if it's correct that the cluster will
continue to work (degraded) if one room is lost?

Will there be any better way to setup such 'stretched' cluster between 2
DCs?  They're extension instead of real DR site...

Sorry for the newbie questions and we'll proceed to have more study and
experiment on this.

Thanks a lot.

> So that any one of following failure won't affect the cluster's 
> operation and data availability:
> 
> any one component in either data center failure of either one of the 
> data center
> 
> Is it possible?
> 
> In general this is possible, but I would consider that replica=2 is 
> not a good idea. In case of a failure scenario or just maintenance and 
> one DC is powered off and just one single disk fails on the other DC, 
> this can already lead to data loss. My advice here would be, if anyhow 
> possible, please don't do replica=2.
> 
> In case one data center failure case, seems replication can't occur any
> more.   Any CRUSH rule can achieve this purpose?
> 
> Sorry for the newbie question.
> 
> Thanks a lot.
> 
> Regards
> 
> /st wong
> 
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