Re: 'btrfs filesystem defragment' makes files explode in size, especially fallocated ones

[Hanabishi <i.r.e.c.c.a.k.u.n+kernel.org@gmail.com>]

On 8/6/24 09:55, Qu Wenruo wrote:

> So either there is something like cgroup involved (which can limits the
> dirty page cache and trigger write backs), or some other weird
> behavior/bugs.

Yes, this line reveals something. I do have modified dirty page cache values. I tend to keep it on low values.

Now playing around with it - yes, it is seems to be the cause. When I tune 'vm.dirty_ratio' and 'vm.dirty_background_ratio' up to higher values, the problem becames less prevalent.

Which means lowering them cranks up the problem to extremes. E.g. try

# sysctl -w vm.dirty_bytes=8192
# sysctl -w vm.dirty_background_ratio=0

With that setup defrag completely obliterates files even with default threshold value.