Re: contraining crush placement possibilities

[lianghaoshen <lianghaoshen@ubuntukylin.com>]

于 2014年03月07日 13:03, Sage Weil 写道:
> On Fri, 7 Mar 2014, Li Wang wrote:
>> Sorry, it is (n/3)*(n/3)*(n/3)/Cn3 = n^3/(27*Cn3)
> Cn3 is "n choose 3"?
>
>>>>> Last night it occurred to me that this is almost just having 
>>>>> pgp_num < pg_num, but I think that's not quite right either.
> Actually, maybe it is.  Basically, say there are X combinations of 3 disks 
> = n choose 3.  Some fraction of these, say Y, are actually used by CRUSH.  
> If we are to reduce that number, that implies that there are some PGs that 
> are overlapping on the same set of disks.  Which more or less reduces to 
> the case where pgp_num < pg_num, or the hashpspool flag isn't set, or any 
> other behavior that makes more than one PG line up on the same disk.  
> Just using fewer PGs in the system, in fact, would help here.  The main 
Dose it mean that we can calculate the pgp_num according to the
reliability request, osd_num and replica_num, instead of using a given
fixed one, namely, 100 pgs/osd ? In fact , when the osd_num of a failure
domain is small , 100pgs can easily cover all of the osds, which means
data lost will occur, when the down osds are in different failure domains.
> problem is that doing this tends to make the distribution less uniform, so 
> there is a tradeoff.
>
> There is a reliability model in ceph-tools.git at
>
> 	https://github.com/ceph/ceph-tools/tree/master/models/reliability
>
> that Mark Kampe built last year.  Sadly I haven't looked at it closely so 
> I'm not sure if it captures this.
>
> sage
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-- 
Best regards,
slhhust


-- 
Best regards,
Lianghao Shen

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