**Describe the bug**
```
5828589:2021.04.27 03:13:08.824665 [ 6101 ] {} <Trace> test_27.concurrent_alter_add_drop_3 (825021f7-129f-4841-ad6b-92215e3f1a94): Renaming temporary part tmp_merge_all_0_4_7_17 to all_0_4_7_17.
5828590:2021.04.27 03:13:08.824778 [ 3358 ] {} <Trace> test_27.concurrent_alter_add_drop_2 (f0a064d0-dc89-4710-9781-a49bb0c16461): Renaming temporary part tmp_merge_all_0_4_7_17 to all_0_4_7_17.
5828591:2021.04.27 03:13:08.824823 [ 6101 ] {} <Warning> test_27.concurrent_alter_add_drop_3 (825021f7-129f-4841-ad6b-92215e3f1a94): Tried to add obsolete part all_0_4_7_17 covered by all_0_4_13_34 (state Committed)
5828623:2021.04.27 03:13:08.824951 [ 3358 ] {} <Warning> test_27.concurrent_alter_add_drop_2 (f0a064d0-dc89-4710-9781-a49bb0c16461): Tried to add obsolete part all_0_4_7_17 covered by all_0_4_13_34 (state Committed)
5828624:2021.04.27 03:13:08.824995 [ 6101 ] {} <Warning> test_27.concurrent_alter_add_drop_3 (825021f7-129f-4841-ad6b-92215e3f1a94) (MergerMutator): Unexpected number of parts removed when adding all_0_4_7_17: 0 instead of 1
5828625:2021.04.27 03:13:08.825147 [ 3358 ] {} <Warning> test_27.concurrent_alter_add_drop_2 (f0a064d0-dc89-4710-9781-a49bb0c16461) (MergerMutator): Unexpected number of parts removed when adding all_0_4_7_17: 0 instead of 1
5828643:2021.04.27 03:13:08.831273 [ 3358 ] {} <Trace> test_27.concurrent_alter_add_drop_2 (f0a064d0-dc89-4710-9781-a49bb0c16461): Trying to immediately remove part all_0_4_7_17 (state Temporary)
5828644:2021.04.27 03:13:08.831658 [ 3358 ] {} <Fatal> : Logical error: 'Part all_0_4_7_17 doesn't exist'.
5828648:2021.04.27 03:13:08.832767 [ 6101 ] {} <Trace> test_27.concurrent_alter_add_drop_3 (825021f7-129f-4841-ad6b-92215e3f1a94): Trying to immediately remove part all_0_4_7_17 (state Temporary)
5828649:2021.04.27 03:13:08.833210 [ 6101 ] {} <Fatal> : Logical error: 'Part all_0_4_7_17 doesn't exist'.

```

https://clickhouse-test-reports.s3.yandex.net/0/9bb4d8769f1064ada4cce6b26d8ff61c11356965/stress_test_(debug).html#fail1
{
            /
            std::shuffle(candidates.begin(), candidates.end(), thread_local_rng);
            String selected_disk_name = candidates.front();
            WriteBufferFromOwnString log_str;
            writeCString("\nbalancer: \n", log_str);
            for (const auto & [disk_name, per_disk_parts] : disk_parts_for_logging)
                writeString(fmt::format("  {}: [{}]\n", disk_name, fmt::join(per_disk_parts, ", ")), log_str);
            LOG_DEBUG(log, log_str.str());
            if (ttl_infos)
                reserved_space = tryReserveSpacePreferringTTLRules(
                    metadata_snapshot,
                    part_size,
                    *ttl_infos,
                    time(nullptr),
                    max_volume_index,
                    is_insert,
                    getStoragePolicy()->getDiskByName(selected_disk_name));
            else
                reserved_space = tryReserveSpace(part_size, getStoragePolicy()->getDiskByName(selected_disk_name));
            if (reserved_space)
            {
                currently_emerging_big_parts.emplace(
                    part_name, EmergingPartInfo{reserved_space->getDisk(0)->getName(), part_info.partition_id, part_size});
                for (const auto & part : covered_parts)
                {
                    if (currently_submerging_big_parts.count(part))
                        LOG_WARNING(log, "currently_submerging_big_parts contains duplicates. JBOD might lose balance");
                    else
                        currently_submerging_big_parts.insert(part);