**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 :)

MatcherFn matcher,
    const StorageMetadataPtr & metadata_snapshot,
    const String & with_name,
    ContextPtr local_context)
{
    String clickhouse_path = fs::canonical(local_context->getPath());
    String default_shadow_path = fs::path(clickhouse_path) / "shadow/";
    fs::create_directories(default_shadow_path);
    auto increment = Increment(fs::path(default_shadow_path) / "increment.txt").get(true);
    const String shadow_path = "shadow/";
    /
    const auto data_parts = getVisibleDataPartsVector(local_context);
    String backup_name = (!with_name.empty() ? escapeForFileName(with_name) : toString(increment));
    String backup_path = fs::path(shadow_path) / backup_name / "";
    for (const auto & disk : getStoragePolicy()->getDisks())
        disk->onFreeze(backup_path);
    PartitionCommandsResultInfo result;
    size_t parts_processed = 0;
    for (const auto & part : data_parts)
    {
        if (!matcher(part->info.partition_id))
            continue;
        LOG_DEBUG(log, "Freezing part {} snapshot will be placed at {}", part->name, backup_path);
        auto data_part_storage = part->data_part_storage;
        String src_part_path = data_part_storage->getRelativePath();
        String backup_part_path = fs::path(backup_path) / relative_data_path;
        if (auto part_in_memory = asInMemoryPart(part))