Swift PUT operation to a specific container is slow

Issue

• PUT operations to a specific container are slower than operations to other containers.
  As one example, PUT operations to a specific container take about 1.2 seconds while PUT operations to other containers take about 0.1 seconds.

• The container has around 4 billion objects

  $ swift stat TestContainer
         Account: AUTH_aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
       Container: TestContainer
         Objects: 3943756627
           :
  

• The container database file size is bigger than 1TB.

  # stat /swift/node/aaa/containers/100/aaa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.db
    File: /swift/node/aaa/containers/100/aaa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.db
    Size: 1099511597056   Blocks: 2147484064 IO Block: 4096   regular file
  Device: xxx/xxxxxx      Inode: 32715434    Links: 1    
  Access: (0600/-rw-------)  Uid: (  xxx/   swift)   Gid: (  xxx/   swift)