Why full compaction skip some groups, after skip those groups ,the tsm files would never be same generation.

When i check my influxdb full compactions, i found some shards were not in same genration after long time when the shards became cold.

I checked codes and found those codes:
```
// Skip the file if it's over the max size and contains a full block and it does not have any tombstones
if len(generations) > 2 && group.size() > uint64(maxTSMFileSize) && c.FileStore.BlockCount(group.files[0].Path, 1) == tsdb.DefaultMaxPointsPerBlock && !group.hasTombstones() {
skip = true
}
```

After my tests, I found that in full compaction, many generations and tsm files were skiped in `Plan(lastWrite time.Time)` funcation. I can't understand those codes.

I want to get all tsm files to become same generation. how?

My tsm files:
```
-rw-r--r-- 1 influxdb influxdb 2148132506 2020-08-03 19:54 000001280-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148116381 2020-08-03 20:02 000001280-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147802956 2020-08-03 20:10 000001280-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147733595 2020-08-03 20:20 000001280-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147596622 2020-08-03 20:28 000001280-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148655600 2020-08-03 20:39 000001280-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 691953357 2020-08-03 20:41 000001280-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148110813 2020-08-04 06:52 000002560-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148035755 2020-08-04 07:02 000002560-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147935765 2020-08-04 07:11 000002560-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147601496 2020-08-04 07:21 000002560-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147778588 2020-08-04 07:28 000002560-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 1915076283 2020-08-04 07:36 000002560-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 2148136116 2020-08-04 18:03 000003840-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148098605 2020-08-04 18:15 000003840-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147823133 2020-08-04 18:23 000003840-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147719099 2020-08-04 18:33 000003840-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147597617 2020-08-04 18:41 000003840-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148651841 2020-08-04 18:50 000003840-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 470553268 2020-08-04 18:53 000003840-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148345443 2020-08-05 03:52 000005120-000000005.tsm
-rw-r--r-- 1 influxdb influxdb 2147848087 2020-08-05 04:01 000005120-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2147950856 2020-08-05 04:10 000005120-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147607675 2020-08-05 04:19 000005120-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2148573966 2020-08-05 04:28 000005120-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 605436892 2020-08-05 04:32 000005120-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148565770 2020-08-05 13:10 000006400-000000005.tsm
-rw-r--r-- 1 influxdb influxdb 2147725139 2020-08-05 13:18 000006400-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2147960884 2020-08-05 13:26 000006400-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147741622 2020-08-05 13:35 000006400-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 1790316401 2020-08-05 13:42 000006400-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2148432151 2020-08-05 22:27 000007680-000000005.tsm
-rw-r--r-- 1 influxdb influxdb 2147775519 2020-08-05 22:36 000007680-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2147954633 2020-08-05 22:45 000007680-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147619041 2020-08-05 22:55 000007680-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2148882643 2020-08-05 23:06 000007680-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 73725070 2020-08-05 23:06 000007680-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148234201 2020-08-06 08:31 000008960-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2147927136 2020-08-06 08:39 000008960-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147957213 2020-08-06 08:50 000008960-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147609291 2020-08-06 08:59 000008960-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2148082864 2020-08-06 09:08 000008960-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 698157143 2020-08-06 09:13 000008960-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 2148423593 2020-08-06 18:13 000010240-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2147930657 2020-08-06 18:23 000010240-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147952789 2020-08-06 18:31 000010240-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147609618 2020-08-06 18:40 000010240-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2148572441 2020-08-06 18:49 000010240-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 537784806 2020-08-06 18:52 000010240-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 2148118579 2020-08-07 05:20 000011520-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148066608 2020-08-07 05:30 000011520-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147856045 2020-08-07 05:39 000011520-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147679025 2020-08-07 05:49 000011520-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147715343 2020-08-07 05:57 000011520-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148838199 2020-08-07 06:07 000011520-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 163030315 2020-08-07 06:08 000011520-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148171773 2020-08-07 16:32 000012800-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148144897 2020-08-07 16:42 000012800-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147811818 2020-08-07 16:51 000012800-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147728326 2020-08-07 17:00 000012800-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147600454 2020-08-07 17:09 000012800-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148656963 2020-08-07 17:18 000012800-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 508354311 2020-08-07 17:21 000012800-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148088209 2020-08-08 04:03 000014080-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148086352 2020-08-08 04:14 000014080-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147803711 2020-08-08 04:23 000014080-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147729753 2020-08-08 04:33 000014080-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147599379 2020-08-08 04:41 000014080-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148610258 2020-08-08 04:49 000014080-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 731104668 2020-08-08 04:53 000014080-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148143693 2020-08-08 15:37 000015360-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148128121 2020-08-08 15:48 000015360-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147753658 2020-08-08 15:55 000015360-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147783088 2020-08-08 16:04 000015360-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147597873 2020-08-08 16:13 000015360-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148447445 2020-08-08 16:22 000015360-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 668247255 2020-08-08 16:25 000015360-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148076518 2020-08-09 03:00 000016640-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148064288 2020-08-09 03:10 000016640-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147816630 2020-08-09 03:18 000016640-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147713590 2020-08-09 03:27 000016640-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147595116 2020-08-09 03:37 000016640-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148626003 2020-08-09 03:45 000016640-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 543205092 2020-08-09 03:47 000016640-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2148118682 2020-08-09 14:21 000017920-000000006.tsm
-rw-r--r-- 1 influxdb influxdb 2148098802 2020-08-09 14:31 000017920-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2147820539 2020-08-09 14:39 000017920-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147716415 2020-08-09 14:48 000017920-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147598759 2020-08-09 14:58 000017920-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2148664200 2020-08-09 15:05 000017920-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 376011071 2020-08-09 15:07 000017920-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2147924029 2020-08-10 17:11 000019896-000000007.tsm
-rw-r--r-- 1 influxdb influxdb 2148048711 2020-08-10 17:21 000019896-000000008.tsm
-rw-r--r-- 1 influxdb influxdb 2147692570 2020-08-10 17:30 000019896-000000009.tsm
-rw-r--r-- 1 influxdb influxdb 2147564022 2020-08-10 17:38 000019896-000000010.tsm
-rw-r--r-- 1 influxdb influxdb 2147889107 2020-08-10 17:50 000019896-000000011.tsm
-rw-r--r-- 1 influxdb influxdb 2147561774 2020-08-10 18:02 000019896-000000012.tsm
-rw-r--r-- 1 influxdb influxdb 2147555846 2020-08-10 18:12 000019896-000000013.tsm
-rw-r--r-- 1 influxdb influxdb 2147734333 2020-08-10 18:19 000019896-000000014.tsm
-rw-r--r-- 1 influxdb influxdb 2148309457 2020-08-10 18:26 000019896-000000015.tsm
-rw-r--r-- 1 influxdb influxdb 1102650622 2020-08-10 18:32 000019896-000000016.tsm
```

```
Influxdb Version: 1.8.0
```

Start by tracing full-compaction planning through Plan(lastWrite time.Time), especially the shown skip condition and FileStore.BlockCount calls. Reproduce the behavior on InfluxDB 1.8 with the reported TSM layout; done means establishing whether skipped groups are expected and identifying a confirmed correction or documented explanation.

Written by the indexing model from the issue text.