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In practice, I saw this on a fully allocated fs with 30TB of space, a 4MiB system chunk (single metadata mode) and ~10k unused data BGs. - The first patch tries to avoid the condition by allocating earlier for the system chunk. - The second patch eagerly commits a single deletion when the system chunk is at risk. More details in the patches themselves. Boris Burkov (2): btrfs: allocate additional SYSTEM space earlier btrfs: commit after deleting an unused block group if system space is low fs/btrfs/block-group.c | 43 +++++++++++++++++++++++++++++++++++++++--- 1 file changed, 40 insertions(+), 3 deletions(-) -- 2.55.0