Re: [PATCH v4 0/4] btrfs: improve stalls under sudden writeback

[David Sterba <dsterba@suse.cz>]

On Thu, Apr 09, 2026 at 10:48:47AM -0700, Boris Burkov wrote:
> If you have a system with very large memory (TiBs) and a normal
> percentage based dirty_ratio/dirty_background_ratio like the defaults of
> 20%/10%, then we can theoretically rack up 100s of GiB of dirty pages
> before doing any writeback. This is further exacerbated if we also see a
> sudden drop in the free memory due to a large allocation. If we
> (relatively likely for a large ram system) also have a large disk, we are
> unlikely to do trigger much preemptive metadata reclaim either.
> 
> Once we do start doing writeback with such a large supply, the results
> are somewhat ugly. The delalloc work generates a huge amount of delayed
> refs without proper reservations which sends the metadata space system
> into a tailspin trying to run yet more delalloc to free space.
> Ultimately, the system stalls waiting for huge amounts of ordered
> extents and delayed refs blocking all users in start_transaction() on
> tickets in reserve_space().
> 
> This patch series aims to address these issues in a relatively targeted
> way by improving our reservations for delalloc delayed refs and by doing
> some very basic smoothing of the work in flush_space(). Further work
> could be done to improve flush_space() heuristics and latency but this
> is already a big help on my observed workloads.
> 
> I was able to reproduce stalls on a more "modest" system with 264GiB of
> ram by using a somewhat silly 80% dirty_ratio.
> 
> I was unfortunately unable to reproduce any stalls on a yet smaller
> system with only 32GiB of ram.
> 
> The first 2 patches do the delayed_ref rsv accounting on btrfs_inode,
> mirroring inode->block_rsv.
> The 3th patch is a cleanup to the types counting max extents
> The 4th patch reduces the size of the unit of work in shrink_delalloc()
> to further reduce stalls.
> ---
> Changelog:
> v4:
> - Treat the extent tree data delayed ref as needing reservation for two cow
>   operations.

As this has been reviewed by Filipe, please add it to for-next.  Thanks.