In my case the issue was that the disk partition allocated for splunk was full. I could detect it by "btrfs fi show" which has indicated "size 1024.00GiB used 1024.00GiB". Usually it should be "size 1024.00GiB used ~500.00GiB" where the second value is less than the first. I solved it by rebalancing with "btrfs balance start -dusage=30".
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