**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 :)

#include <Storages/MergeTree/MergeTreeWriteAheadLog.h>
#include <Storages/MergeTree/MergeTreeDataPartInMemory.h>
#include <Storages/MergeTree/MergeTreeData.h>
#include <Storages/MergeTree/MergeTreeDataWriter.h>
#include <Storages/MergeTree/MergedBlockOutputStream.h>
#include <IO/MemoryReadWriteBuffer.h>
#include <IO/ReadHelpers.h>
#include <IO/copyData.h>
#include <Interpreters/Context.h>
#include <Poco/JSON/JSON.h>
#include <Poco/JSON/Object.h>
#include <Poco/JSON/Stringifier.h>
#include <Poco/JSON/Parser.h>
#include "Storages/MergeTree/DataPartStorageOnDisk.h"
#include <sys/time.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int UNKNOWN_FORMAT_VERSION;
    extern const int CANNOT_READ_ALL_DATA;
    extern const int BAD_DATA_PART_NAME;
    extern const int CORRUPTED_DATA;
}
MergeTreeWriteAheadLog::MergeTreeWriteAheadLog(