Discuss delete storage group SQL statement

[Haiming Zhu <he...@gmail.com>]

Hi,

After reviewing the code of delete storage group SQL statement, i have some
questions about the implementation.

First, when deleting several storage groups in one statement, the metadata
in MManger will be deleted together after all storage groups are deleted.
But, if there are concurrent write operations, the write operations will
use old metadata, after the 'delete storage group' statement is executed
successfully and the metadata is deleted from MManger, the data written in
this period cannot be queried any more.

Secondly, processorMap in StorageEngine class may need a
ReentrantReadWriteLock. For example, if we execute delete storage group
statement in cli and there are concurrent write operations to these storage
groups. Current code first remove storage group from processorMap and then
delete it, so other write operations will create a new
VirtualStorageGroupManager in processorMap and the new VSGManager's recover
method will conflict with the delete procedure, because tsfile may be
truncated in recover and delete needs flushing memtable to the same tsfile.
However, if we first delete a storage group and then remove it from
processorMap, other write operations may create a new TsFileProcessor in
the interval, making this delete statement incorrect. So, we need a
ReentrantReadWriteLock to lock processorMap.

Best,
------------------------------------
Haiming Zhu
School of Software, Tsinghua University

朱海铭
清华大学 软件学院