Hi,

In Jira 110[1] and 118[2], users describe a scene that when the disk is full, 
IoTDB still allows creating metadata and inserting data, which is unreasonable.


Therefore, I add a parameter "readOnly" in IoTDBConfig to denote the 
"read-only" mode of IoTDB in PR[3] (already merged). When the current mode is 
"read-only", IoTDB only serves queries and reject other non-query operations 
(Modifying of data and metadata, async flushing WAL).


This property will be set true in the following scenes:


- meeting error in creating TsFile processor.
- meeting error in flushing a memtable to disk.
- meeting error in flushing TsFileMetaData to disk.
- meeting error in flushing WAL to disk.


There are also some scenes that the readOnly be set false:


- Starting IoTDB.
- ... (need more strategies to recover from read-only mode)




[1] 
https://issues.apache.org/jira/projects/IOTDB/issues/IOTDB-110?filter=allopenissues
[2] 
https://issues.apache.org/jira/projects/IOTDB/issues/IOTDB-118?filter=allopenissues
[3] https://github.com/apache/incubator-iotdb/pull/245


Best,
--
Jialin Qiao
School of Software, Tsinghua University


乔嘉林
清华大学 软件学院

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