When Client try to read(SELECT) on which all ready open to Write(INSERT/DELETE)
1)Client request for READ a block from HDFS file system.
2) That given block is all ready open for write and it wait till WRITE Operation complete, because its Start/End block ID will change during Write, hence Client read wait till complete.
3) Client wait till "dfs.client.failover.max.attempts" in HDFS-SITE.xml , Ex:- 10 attempt , it try for 10 attempt to read the operation , mean time if Write HDFS complete , client will read & complete.
4) if Client not able to read within max "dfs.client.failover.max.attempts" attempt Client read request will fail.