Re: What the ceph do when detecting one OSD overloaded?

[Wido den Hollander <wido@42on.com>]

On 06/06/2014 11:27 AM, zou wonder wrote:
> Hi Wido:
>
>     Thanks for your kindness, i checked the codes just now, seems there
> were no overload related thing in CRUSH codes. It is a little bit not
> consistent with the CRUSH paper.
>   So if the %100 utilized OSD is returned, the objects will still be
> written on the underlined storage device?
>

Yes. If the OSD is up/in it will be selected by CRUSH and data will be 
read from it and written to it.

Again, having one OSD being 100% utilized and the rest 40% is not 
something you'll see very often since you stripe data over objects.

> How about the device failure case? All the objects on the failure
> device will be replicated to the good ones? once the device is
> recoverd, the data will be replicated back?

When the OSD fails recovery will kick in after 5 minutes and the data 
will find a new location.

If the OSD comes back, the data goes back to that OSD.

>
> Best Regards
> Zou
>
> On Fri, Jun 6, 2014 at 4:27 PM, Wido den Hollander <wido@42on.com> wrote:
>> On 06/06/2014 10:14 AM, zou wonder wrote:
>>>
>>> Hi buddies,
>>>
>>>     I am doing investigation on Ceph and Swift,and I am newbie to Ceph.
>>>
>>> I am unclear to the behaviour of the Ceph when there is overload
>>> situation.
>>> According to the doc, when Ceph do CRUSH, if it find the OSD is overload,
>>> it will skip it to select the other OSD. It means if the OSD don't get
>>> overloaded
>>> the object should be put in this OSD. So how about the original on put on
>>> this OSD when it is not get overloaded?
>>> Can we read it ? When doing CRUSH , the OSD will be skip.
>>>
>>
>> CRUSH will not take any performance characteristics into account. If a OSD
>> is 100% utilized it will still be selected by CRUSH.
>>
>> Keep in mind however that Block Devices are striped in 4MB chunks and the
>> same happens for Objects stored via the RADOS Gateway.
>>
>>> If my understanding is wrong, please correct me.
>>>
>>> Best Regards,
>>> zou
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>
>>
>>
>> --
>> Wido den Hollander
>> 42on B.V.
>> Ceph trainer and consultant
>>
>> Phone: +31 (0)20 700 9902
>> Skype: contact42on


-- 
Wido den Hollander
42on B.V.
Ceph trainer and consultant

Phone: +31 (0)20 700 9902
Skype: contact42on