Ceph » rgw

Background：
Ceph version 12.2.4 (52085d5249a80c5f5121a76d6288429f35e4e77b) luminous (stable)
Index pool is on ssd.
There is a very big bucket with more than 10 million object and 500TB data.
Ceph health is OK.¶

Describe:
When use s3 list_object() to list, some uploaded object can not be listed and some uploaded object have an old lastModified time.
But at the same time, we can get this object by an exact key. And if I put a new object into this bucket, it can be listed.
It seems that some indexes during a period of time have been lost.¶

Some else：
I found that one bucket will have many indexes, and we can use "radosgw-admin metadata list bucket.instance | grep "{bucket name}" to show them. But I can not found a doc to describe this feature. And we can use "adosgw-admin bucket stats --bucket {bucket_name}" to get id as the active instance id.
I use "rados listomapkeys" at active index to get all object in a index, it is really lost. But when I use "rados listomapkeys" at another index which is not active as mentioned above, I found the lost object index.¶

Question:
Why my index lost?
How to recover？
Why radosgw has many index instances？