Multiple (parallel clients) `cp` with the same files (using `-u`) failed
cp: cannot create regular file '/home/bsc25/bsc25015/test/mnt/Alya.x': File exists
gkfs_daemon: /home/bsc25/bsc25015/GEKKO/gekkofs/src/daemon/backend/metadata/merge.cpp:148: virtual bool MetadataMergeOperator::FullMergeV2(const rocksdb::MergeOperator::MergeOperationInput&, rocksdb::MergeOperator::MergeOperationOutput*) const: Assertion op.size < fsize failed.