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dsl_dir_transfer_space() is mostly called after dsl_dir_diduse_space(), which already calls dmu_buf_will_dirty() for the same dbuf and tx, so its duplicate call in those cases will change nothing, only spend time. Skipping this call by four times reduces time spent in dbuf_write_done() and descendants, updating dataset statistics with several congested lock acquisitions. When rewriting 8K zvol blocks at 1GB/s rate, this reduces CPU time spent inside dbuf_write_done(), according to profiling, from 45% of 683K samples to 18% of 422K. MFC after: 2 weeks
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