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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/10/2 上午9:30, Wang Yugui wrote:
> Hi, Qu Wenruo
> 
>> On 2020/10/1 下午11:36, Wang Yugui wrote:
>>> Hi, Qu Wenruo
>>>
>>> Chunk allocator like calculation will get the right value, but it is
>>> slow for big file system such as 500T.
>>
>> Nope, the ballon allocator doesn't have any size limit, thus it will try
>> to use as much space as possible in a single run.
>>
>> If the 500T fs only has say 100T used, and the remaining 400T is split
>> into, say 2 parts, then just two small run would finish.
>>
>> On the other hand, if the 500T is mostly used, only 100T unallocated and
>> the 100T is in a big unallocate chunk (under most cases it's true), then
>> just one allocation run is enough.
>>
>> So in short, it's unrelated to the fs, but how fragmented the
>> unallocated space is.
>> And normally unallocated space is not fragmented at all, thus it's very
>> speedy.
> 
> We need some more complex example for 'Chunk allocator like calculation'
> to make it easy to understand.
> 
> 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.

- 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
> 


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