The newbie questions

The newbie questions

[RomanS]

Hello.

I'm not sure that this list is correct to ask the following question.
It would be nice to understand the architecture of Ceph. To do this i
want to find answers for the following questions:

1. Ceph supports 2 types of scrubbing. To do check need to calculate
something and compare the result and the previously stored value. If i
understand correctly Ceph uses crc32. Where the value is stored?

2. Where can i read (in details) about data placement (structure)? only sources?

3. I tried to find information about behavior of Ceph in case of
failures?  For example I put a data to Ceph. Placement logic has done
its work and we know where need to place the data. We send data to
primary OSD and it resend the data to the secondary OSDs. At some
point a primary OSD fails. What next? Need to call CRUSH again?

4. What is the minimal structure of data in Ceph? object or object-chunk?

5. Where object-metadata is stored? as a separate metadata-object or
as a part of object structure?

Thanks.

Re: The newbie questions

[kefu chai]

On Fri, Jul 1, 2016 at 5:15 AM, RomanS <execooler@gmail.com> wrote:
> Hello.
>
> I'm not sure that this list is correct to ask the following question.
> It would be nice to understand the architecture of Ceph. To do this i
> want to find answers for the following questions:
>
> 1. Ceph supports 2 types of scrubbing. To do check need to calculate
> something and compare the result and the previously stored value. If i
> understand correctly Ceph uses crc32. Where the value is stored?

the previously stored values are stored in memory. and we scrub the objects
in batch/chunk, see PG::chunky_scrub().

>
> 2. Where can i read (in details) about data placement (structure)? only sources?

source, and we have some document for explaining the internals of
Ceph, see http://docs.ceph.com/docs/master/dev/. and you might want to
checkout the Ceph over youtube, see
 https://www.youtube.com/channel/UCno-Fry25FJ7B4RycCxOtfw. and
papers on Ceph and CRUSH, see http://ceph.com/resources/.

>
> 3. I tried to find information about behavior of Ceph in case of
> failures?  For example I put a data to Ceph. Placement logic has done
> its work and we know where need to place the data. We send data to
> primary OSD and it resend the data to the secondary OSDs. At some
> point a primary OSD fails. What next? Need to call CRUSH again?

see http://docs.ceph.com/docs/master/dev/peering/.

>
> 4. What is the minimal structure of data in Ceph? object or object-chunk?

i think it's object. as RADOS operations are object centric.

>
> 5. Where object-metadata is stored? as a separate metadata-object or
> as a part of object structure?

regarding to FileStore, xattr is stored in the xattr of the file where
the object
is stored in. and omap is stored in the KV store on that OSD.


>
> Thanks.
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-- 
Regards
Kefu Chai