Re: Report correct filesystem usage / limits on BTRFS subvolumes with quota

[Andrei Borzenkov <arvidjaar@gmail.com>]

10.08.2018 12:33, Tomasz Pala пишет:
> 
>> For 4 disk with 1T free space each, if you're using RAID5 for data, then
>> you can write 3T data.
>> But if you're also using RAID10 for metadata, and you're using default
>> inline, we can use small files to fill the free space, resulting 2T
>> available space.
>>
>> So in this case how would you calculate the free space? 3T or 2T or
>> anything between them?
> 
> The answear is pretty simple: 3T. Rationale:
> - this is the space I do can put in a single data stream,
> - people are aware that there is metadata overhead with any object;
>   after all, metadata are also data,
> - while filling the fs with small files the free space available would
>   self-adjust after every single file put, so after uploading 1T of such
>   files the df should report 1.5T free. There would be nothing weird(er
>   that now) that 1T of data has actually eaten 1.5T of storage.
> 
> No crystal ball calculations, just KISS; since one _can_ put 3T file
> (non sparse, uncompressible, bulk written) on a filesystem, the free space is 3T.
> 

As far as I can tell, that is exactly what "df" reports now. "btrfs fi
us" will tell you both max (reported by "df") and worst case min.