**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

MergeTreeData & storage_,
    const DiskPtr & disk_,
    const String & name_)
    : storage(storage_)
    , disk(disk_)
    , name(name_)
    , path(storage.getRelativeDataPath() + name_)
    , pool(storage.getContext()->getSchedulePool())
    , log(&Poco::Logger::get(storage.getLogName() + " (WriteAheadLog)"))
{
    init();
    sync_task = pool.createTask("MergeTreeWriteAheadLog::sync", [this]
    {
        std::lock_guard lock(write_mutex);
        out->sync();
        sync_scheduled = false;
        sync_cv.notify_all();
    });
}
MergeTreeWriteAheadLog::~MergeTreeWriteAheadLog()