debezium 抽取大表优化

现象

　　在debezium 抽取 千万级大表时，发现snapshot时同步速率在 2s 10000 row ，同时还有 young GC 信息打印

原因分析

网络原因

　　首先排除网络延迟的原因，ping 一下目的主机，发现 延迟在0.1ms

本身原因

　　查看SnapshotReader源码

// Scan the rows in the table ...
                            long start = clock.currentTimeInMillis();
                            logger.info("Step {}: - scanning table '{}' ({} of {} tables)", step, tableId, ++counter, capturedTableIds.size());

                            Map<TableId, String> selectOverrides = context.getConnectorConfig().getSnapshotSelectOverridesByTable();

                            String selectStatement = selectOverrides.getOrDefault(tableId, "SELECT * FROM " + quote(tableId));
                            logger.info("For table '{}' using select statement: '{}'", tableId, selectStatement);
                            sql.set(selectStatement);

                            try {
                                int stepNum = step;
                                mysql.query(sql.get(), statementFactory, rs -> {
                                    try {
                                        // The table is included in the connector's filters, so process all of the table records
                                        // ...
                                        final Table table = schema.tableFor(tableId);
                                        final int numColumns = table.columns().size();
                                        final Object[] row = new Object[numColumns];
                                        while (rs.next()) {
                                            for (int i = 0, j = 1; i != numColumns; ++i, ++j) {
                                                Column actualColumn = table.columns().get(i);
                                                row[i] = readField(rs, j, actualColumn, table);
                                            }
                                            recorder.recordRow(recordMaker, row, clock.currentTimeAsInstant()); // has no row number!
                                            rowNum.incrementAndGet();
                                            if (rowNum.get() % 100 == 0 && !isRunning()) {
                                                // We've stopped running ...
                                                break;
                                            }
                                            if (rowNum.get() % 10_000 == 0) {
                                                if (logger.isInfoEnabled()) {
                                                    long stop = clock.currentTimeInMillis();
                                                    logger.info("Step {}: - {} of {} rows scanned from table '{}' after {}",
                                                            stepNum, rowNum, rowCountStr, tableId, Strings.duration(stop - start));
                                                }
                                                metrics.rowsScanned(tableId, rowNum.get());
                                            }
                                        }

,原来它默认是对表做一个select * ，然后在内存中对 整个表做 个 count ，之后迭代发送数据

解决方案

在官网里找snapshot 的配置，发现一个参数 min.row.count.to.stream.results

https://debezium.io/documentation/reference/1.4/connectors/mysql.html#mysql-property-min-row-count-to-stream-results

 ，尝试配了这个参数后，日志打印没有去select * ，直接 每批次 发送10000，速度提升了20倍。