[RFC] Add an option to disable automatic chunk reclamation.

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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Since upgrading to a kernel with the automatic chunk reclamation 
patches, I've noticed a number of issues with BTRFS that all seem to 
either be caused by, or are further exacerbated by, this 'feature'.

The four big issues I've seen regarding it are:
1. TRIM/DISCARD support is broken as a (partial?) result of this.
2. It appears to expose underlying issues with the defrag code (stuff 
ending up more fragmented after defrag).
3. Since upgrading to a kernel with this patch, most of the BTRFS 
filesystems that I have that are very rewrite heavy have gotten very 
noticeably slower that they were beforehand (and this goes away when I 
run them on a kernel without auto-reclaim).
4. All of my filesystems are experiencing seemingly non-deterministic 
delays around most large scale VFS level operations (eg, deleting or 
relocating lots of files)

While I understand that this feature does solve (at least partially) an 
very real issue with BTRFS, there were a number of people who never had 
this issue to begin with because we ran regular balance operations on 
our filesystems.

Based on this, I would like to propose that some method be provided to 
disable auto-reclaim be added.  Personally, I would prefer to leave it 
as the default and have a mount option to disable it.


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