Re: btrfs space used issue

vinayak hegde posted on Thu, 01 Mar 2018 14:56:46 +0530 as excerpted:

This will happen over and over again until we have completely
overwritten the original extent, at which point your space usage will go
back down to ~302g.We split big extents with cow, so unless you've got
lots of space to spare or are going to use nodatacow you should probably
not pre-allocate virt images

Indeed.  Preallocation with COW doesn't make the sense it does on an 
overwrite-in-place filesystem.  Either nocow it and take the penalties 
that brings[1], or configure your app not to preallocate in the first 
place[2].

---
[1] On btrfs, nocow implies no checksumming or transparent compression, 
either.  Also, the nocow attribute needs to be set on the empty file, 
with the easiest way to do that being to set it on the parent directory 
before file creation, so it's inherited by any newly created files/
subdirs within it.

[2] Many apps that preallocate by default have an option to turn 
preallocation off.

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