Re: simple Chunk allocator like calculation to replace Factor based calculation

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

We have another user case difficult to process.

Use case: 
Add 10T disk * 4  to near full  full RAID10 10T *4;
free space maybe be such as 10T,10T,10T,10T,2G,2G,2G,2G.

There maybe a lot of iterations for this case because of 2G chunk size,
and then result in bad performance?

Best Regards
王玉贵
2020/10/02

> 
> 
> On 2020/10/2 上午9:59, Wang Yugui wrote:
> > Hi,
> > 
> > 
> >>> such as
> >>> 1) RAID10 with 8T,1T,1T,1T,1T
> >>>     the virtal chunk size of 1st iteration:	1T or 0.33T?
> >>>     1T    chunk will use 4T at most
> >>>     0.33T chunk will use 5.33T at most?
> >>
> >> You didn't get the point.
> >> For the each loop we:
> >> - Sort the devices with their free space.
> >>   In this case, it's 8, 1, 1, 1, 1.
> >>
> >> - Round down to dev increament
> >>   Then we got 8, 1, 1, 1.
> >>
> >> - Then allocate chunk
> >>   Since the biggest unallocated space is 1T, we allocate a RAID10 with
> >>   1T stripe size, which will be a 2T chunk.
> >>
> >>   The remaining size is 7, 0, 0, 0, 1.
> > 
> > That is the problem.  1T chunk size is too big for this case.
> > 
> > if we use 1/3T chunk size, the result will be same as 1G chunk size.
> > 
> > 1st:	8T - 1/3T     2/3T, 2/3T, 2/3T, 1T
> > 2nd:	8T -2/3T     2/3T, 1/3T, 1/3T, 2/3T
> > 3rd:	8T -1T        1/3T, 1/3T, 0T, 1/3T
> > 4th:	8T -4/3T     0T,     0T,   0T,  0T
> 
> You're right, smaller balloon chunk size would make the allocation more
> accurate to real chunk allocator.
> 
> However that would slow down the calculation, don't forget that we need
> to run that calculation on each chunk allocation.
> Changing the balloon chunk allocation size dynamically may improve this.
> 
> But please also keep in mind that, with less and less space left, our
> predication will be more and more accurate, and under estimate is always
> less a problem.
> 
> So I'll keep your suggestion for future enhancement.
> 
> Thanks for pointing out the pitfall of current calculation,
> Qu
> 
> > 
> > Best Regards
> > 王玉贵
> > 2020/10/02
> > 
> >>
> >> - We go to next round.
> >>   No way to allocate new chunk.
> >>
> >> In this case, we can only get 2T chunk.
> >> Just as the chunk allocator do.
> >>
> >>>
> >>> 2) RAID10 with 8T,1T,1T,1T,0.5T
> >>>     the virtal chunk size of 1st iteration:0.5T or smaller?
> >>
> >> Still the same, 2T chunk can be allocated using the largest 4 devices.
> >>
> >> Thanks,
> >> Qu
> >>
> >>>
> >>> Best Regards
> >>> 王玉贵
> >>> 2020/10/02
> >>>
> >>>
> >>> --------------------------------------
> >>> 北京京垓科技有限公司
> >>> 王玉贵	wangyugui@e16-tech.com
> >>> 电话：+86-136-71123776
> >>>
> >>
> > 
> > --------------------------------------
> > 北京京垓科技有限公司
> > 王玉贵	wangyugui@e16-tech.com
> > 电话：+86-136-71123776
> > 
> 

--------------------------------------
北京京垓科技有限公司
王玉贵	wangyugui@e16-tech.com
电话：+86-136-71123776