Re: Why does defragmenting break reflinks?

[Ferry Toth <fntoth@gmail.com>]

Op 30-05-2025 om 02:27 schreef Qu Wenruo:
> 
> 
> 在 2025/5/30 09:22, 𝕍𝕖𝕝𝕠𝕔𝕚𝕗𝕪𝕖𝕣 写道:
>> BTRFS-FILESYSTEM(8) says "defragmenting  with  Linux kernel versions < 
>> 3.9 or ≥ 3.14-rc2 as well as with Linux stable kernel versions ≥ 
>> 3.10.31, ≥ 3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data 
>> (for example files copied with cp --reflink, snapshots or de- 
>> duplicated data)." Why does defragmenting not preserve reflinks and 
>> why was it removed?
>>
> 
> Defrag means to re-dirty the data, and write them back again, which will 
> cause COW.

That sounds like an implementation choice.

> And by nature this breaks reflinked data extents.

With a different implementation this would not by nature be the case.

Actually this makes defrag a very dangerous operation on disks with many 
snapshots (> 20). When you would defrag each snapshot suddenly your 5% 
full disk would be 100%.