**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 :)

class MergeTreePartsMover;
class MergeTreeDataMergerMutator;
class MutationCommands;
class Context;
using PartitionIdToMaxBlock = std::unordered_map<String, Int64>;
struct JobAndPool;
class MergeTreeTransaction;
struct ZeroCopyLock;
class IBackupEntry;
using BackupEntries = std::vector<std::pair<String, std::shared_ptr<const IBackupEntry>>>;
class MergeTreeTransaction;
using MergeTreeTransactionPtr = std::shared_ptr<MergeTreeTransaction>;
struct EmergingPartInfo
{
    String disk_name;
    String partition_id;
    size_t estimate_bytes;
};
struct CurrentlySubmergingEmergingTagger;
struct SelectQueryOptions;
class ExpressionActions;
using ExpressionActionsPtr = std::shared_ptr<ExpressionActions>;
using ManyExpressionActions = std::vector<ExpressionActionsPtr>;
class MergeTreeDeduplicationLog;