I have a collection contains about 4000 documents and another collection contains about 310000 documents.on this db,it has about 14 collections,and the max collection contains about 310000 documents. I use the tool mongodump to back up my database from server and restore to my local machine for this test. my server is linux,the dump command is: /usr/local/mongodb/mongodump --out=/home/databak/mongo/201012301639/ my local machine is windows 7,the restore command is: mongorestore --host=127.0.0.1 --db=myrestoretest --drop G:\Media\myrestoretest after restore finished.I found that the index on the collection contains about 4000 documents and the other collection contains about 310000 documents lost.