version : 10.1 - 11.2        
During node join and leave (reconfiguration) in a cluster we need to reconfigure, in that particular case we use Short Disk TimeOut (SDTO) which is in all versions SDTO = MC – reboottime (usually 3 seconds)

来自  CSS Timeout Computation in Oracle Clusterware (文档 ID 294430.1)   

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If a heartbeat failed between the nodes, voting disk write now should complete within SDTO. This is called shortdiskTimeout. There are two disk out parameters used in Oracle Clusterware which are: longdisktimeout (LDTO) and shortdisktimeout (SDTO).
The voting disk write should complete within SDTO when there is a “network heart beat failure”. When there is network heart beat failure, Oracle Clusterware no longer considers the LDTO.  

来自  http://www.orafaq.com/node/2504   

以上关于SDTO及LDTO的说法， 可以总结为如下几句:  

SDTO  -- 节点加入或删除时cluster需进行reconfiguration的时间，SDTO=MC-rebootime=30-3=27秒 。
            或如果在节点之间的心跳failed，voting disk写操作应该在SDTO(27秒)时间内完成 。

LDTO  -- 是指正常的RAC操作中允许Votedisk I/O完成超时的时间，为200秒。也就是说， 如果网络心跳failed ,  那么只会使用到 SDTO ,  
             而不会使用到 LDTO ,  LDTO 这个 200秒 只是在网络心跳正常，进行正常RAC操作时候才会用到 。