Re: Feature requests: online backup - defrag - change RAID level

[General Zed <general-zed@zedlx.com>]

Quoting Chris Murphy <lists@colorremedies.com>:

> On Thu, Sep 12, 2019 at 1:18 PM <webmaster@zedlx.com> wrote:
>>
>> It is normal and common for defrag operation to use some disk space
>> while it is running. I estimate that a reasonable limit would be to
>> use up to 1% of total partition size. So, if a partition size is 100
>> GB, the defrag can use 1 GB. Lets call this "defrag operation space".
>
> The simplest case of a file with no shared extents, the minimum free
> space should be set to the potential maximum rewrite of the file, i.e.
> 100% of the file size. Since Btrfs is COW, the entire operation must
> succeed or fail, no possibility of an ambiguous in between state, and
> this does apply to defragment.
>
> So if you're defragging a 10GiB file, you need 10GiB minimum free
> space to COW those extents to a new, mostly contiguous, set of exents,

False.

You can defragment just 1 GB of that file, and then just write out to  
disk (in new extents) an entire new version of b-trees.
Of course, you don't really need to do all that, as usually only a  
small part of the b-trees need to be updated.

The only problem that there can arise is if the original file is  
entirely one 10 GB extent. In that case, that extent should be split  
into smaller parts. If btrfs cannot do that splitting, mostly (but not  
entirely) in-place, than it is a fundamentally flawed design, but I  
reckon that is not the case. You would have to be really stupid to  
design a filesystem that way, if it is even possible to design a  
filesystem with such property.