按官方文档配置了1个gtm节点(机器1)，2个coordinator&datanode节点(机器2，3),1个备份节点(机器4)
参考文章：Creating a Postgres-XL cluster
相关命令：
add coordinator master coord1 DB60 30001 30011 $dataDirRoot/coord_master.1 my_conf/coordExtraConfig my_conf/pgHbaConfig
add datanode master dn1 DB60 40001 40011 $dataDirRoot/dn_master.1 none my_conf/datanodeExtraConfig my_conf/pgHbaConfig
add coordinator master coord2 DB61 30001 30011 $dataDirRoot/coord_master.2 my_conf/coordExtraConfig my_conf/pgHbaConfig
add datanode master dn2 DB61 40001 40011 $dataDirRoot/dn_master.2 none my_conf/datanodeExtraConfig my_conf/pgHbaConfig
add datanode slave dn1 DB62 40101 40111 $dataDirRoot/dn_slave.1 none $dataDirRoot/datanode_archlog.1添加slave节点后，slave节点的日志如下：
LOG: database system was interrupted; last known up at 2016-02-29 15:49:04 CST
FATAL: the database system is starting up
FATAL: the database system is starting up
LOG: entering standby mode
LOG: restored log file "000000010000000000000089" from archive
LOG: redo starts at 0/89000028
LOG: consistent recovery state reached at 0/890000F8
LOG: database system is ready to accept read only connections
cp: cannot stat ‘/home/postgres/DATA/pgxl/nodes/datanode_archlog.1/00000001000000000000008A’: No such file or directory
LOG: started streaming WAL from primary at 0/8A000000 on timeline 1
FATAL: Was unable to obtain a snapshot from GTM.
FATAL: Was unable to obtain a snapshot from GTM.
FATAL: Was unable to obtain a snapshot from GTM.
GTM节点报如下错误：
1:140050603951872:2016-02-29 15:49:02.107 CST -LOG: GTM_ERRCODE_TOO_OLD_XMIN - node_name dn1, reported_xmin 782028, previously reported_xmin 882982, GTM_GlobalXmin 882982
LOCATION: GTM_HandleGlobalXmin, register_common.c:1042
只是备份异常，其他DML都正常。
有了解的吗？求点拨。
另，以上配置是参考官方文档摸索着配的，不知道是不是有问题，希望配置成功的前辈，分享一下经验，谢谢了。
写回答
第一次注册成功后，获得的Xmin是globalxmin，可能会有这个报错存在。你跑几个事务后再看看
/*
* The remote node must not report a xmin which precedes the GTM_GlobalXmin
* we have already computed. If it ever happens, send an error back and let
* the remote node handle it, possibly restarting itself
*/
if (GlobalTransactionIdPrecedes(reported_xmin, GTM_GlobalXmin))
{
*errcode = GTM_ERRCODE_TOO_OLD_XMIN;
mynodeinfo->joining = true;
/*
* When node registers from the first time, the reported_xmin is set
* to GTM_GlobalXmin and what we receive from the node would most
* likely precedes that, especially because nodes' latestCompletedXid
* could precede our GTM_GlobalXmin. The node is prepared to handle
* that case, but we should avoid logging an useless and often
* confusing log message.
*/
if (mynodeinfo->reported_xmin_time)
elog(LOG, "GTM_ERRCODE_TOO_OLD_XMIN - node_name %s, reported_xmin %d, "
"previously reported_xmin %d, GTM_GlobalXmin %d", node_name,
reported_xmin, mynodeinfo->reported_xmin, GTM_GlobalXmin);
mynodeinfo->reported_xmin_time = GTM_TimestampGetCurrent();
mynodeinfo->reported_xmin = GTM_GlobalXmin;
GTM_RWLockRelease(&mynodeinfo->node_lock);
return InvalidGlobalTransactionId;
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