Allocation groups also help to optimize parallel IO performance on multiprocessor systems, because more than one metadata update can be "in transit" at the same time.
分配组还有助于在多处理器系统上优化并行io性能，因为可以同时有多个元数据更新处于“在传输中”。
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Thanks to allocation groups, the XFS code will allow multiple threads and processes to continue to run in parallel, even if many of them are performing non-trivial IO on the same filesystem.