Why Does an 'ls' Command Executed from a 'vsftpd' Connection on a Gluster Volume with 200k+ Files Take Several Seconds?

Issue

• An lscommand executed on a Gluster Filesystem with 200K entries returns immediately. Let's assume a standard Gluster mount:

  localhost:/sample_volname on /test type fuse.glusterfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other,max_read=131072)
  

  and approximately 200K files created inside this mount:

  ls /test | wc -l
  237329
  

  A plain lscommand returns in no time:

  time ls /test/file111111
  /test/file111111
  
  real    0m0.049s
  user    0m0.001s
   sys    0m0.001s
  

  However, when doing a remote connection to this file system using a vsftpdclient, the same lscommand returns in a few seconds:

  ftp gluster-1
  Connected to gluster-1 .
  220 (vsFTPd 3.0.2)
  Name (gluster-1:root): 
  331 Please specify the password.
  Password: 
  230 Login successful.
  Remote system type is UNIX.
  Using binary mode to transfer files.
  
  ftp> ls /test/file111111
  227 Entering Passive Mode.
  150 Here comes the directory listing.
  -rw-r--r--    1 0        0               0 Oct 21 11:22 file111111
  226 Directory send OK.
  

  From the vsftpd logs:

  Mon Oct 21 10:48:09 2019 [pid 29143] [test] FTP response: "150 Here comes the directory listing."
  Mon Oct 21 10:48:23 2019 [pid 29143] [test] FTP response:  "226 Directory send OK."
  

  The command here took 14 seconds. This time increases if the volume layout contains a higher number of bricks or if the number of files stored is also bigger.

• Why is this difference observed?