为提高效率，提问时请提供以下信息，问题描述清晰可优先响应。
- 【TiDB 版本】：v3.0.9
- 【问题描述】：dm 同步过程中遇到了这个问题 “errors”: [ { “Type”: “UnknownError”, “msg”: “[code=30014:class=relay-unit:scope=upstream:level=high] start reader for UUID 4a017335-b2a1-11e9-afac-506b4bff3dfc.000001: start sync from GTID set 209ce98b-6584-11e9-b8d3-506b4bfdd244:1-36395787722,4a017335-b2a1-11e9-afac-506b4bff3dfc:1-91722830456,eeb12265-9d4b-11e8-9f3a-506b4bbd5c96:1-1223451434,f2b738b2-c3eb-11e8-9b18-506b4b2319d8:1-23072940722: dial tcp: i/o timeout\ngithub.com/pingcap/dm/pkg/terror.(*Error).Delegate\ \t/go/src/github.com/pingcap/dm/pkg/terror/terror.go:267\ngithub.com/pingcap/dm/pkg/binlog/reader.(*TCPReader).StartSyncByGTID\ \t/go/src/github.com/pingcap/dm/pkg/binlog/reader/tcp.go:102\ngithub.com/pingcap/dm/relay/reader.(*reader).setUpReaderByGTID\ \t/go/src/github.com/pingcap/dm/relay/reader/reader.go:160\ngithub.com/pingcap/dm/relay/reader.(*reader).Start\ \t/go/src/github.com/pingcap/dm/relay/reader/reader.go:109\ngithub.com/pingcap/dm/relay.(*Relay).setUpReader\ \t/go/src/github.com/pingcap/dm/relay/relay.go:606\ngithub.com/pingcap/dm/relay.(*Relay).process\ \t/go/src/github.com/pingcap/dm/relay/relay.go:304\ngithub.com/pingcap/dm/relay.(*Relay).Process\ \t/go/src/github.com/pingcap/dm/relay/relay.go:191\ngithub.com/pingcap/dm/dm/worker.(*realRelayHolder).run\ \t/go/src/github.com/pingcap/dm/dm/worker/relay.go:167\ngithub.com/pingcap/dm/dm/worker.(*realRelayHolder).Start.func1\ \t/go/src/github.com/pingcap/dm/dm/worker/relay.go:143\ runtime.goexit\ \t/usr/local/go/src/runtime/asm_amd64.s:1357”, “error”: null } 若提问为性能优化、故障排查类问题，请下载脚本运行。终端输出的打印结果，请务必全选并复制粘贴上传。