[Dovecot] Resynchronization of indexes (or Warning: Maildir Scanning <dir> took <X> seconds)

[Angel L. Mateo]

Hello,

	We have a dovecot servers farm with storage in NFS and indexes in local 
files. Under heavy load conditions we a log of messages in the form:

Sep  3 12:35:49 myotis32 dovecot: imap(<imapuser>): Warning: Maildir: 
Scanning <maildir cur directory> took 62 seconds (9516 readdir()s, 0 
rename()s to cur/, why=0x80)

	We have check that messages are directed with lmtp to the same server 
the user (scenario 3 at http://wiki2.dovecot.org/NFS with director in 
front of backend servers).

	So my question is why is dovecot using these readdirs operations? why 
it is scanning the whole cur directory of the user? I guess this is an 
index resynchronization, but I don't know could it be the cause.

PS: I have attached my dovecot -n output
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# 2.1.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.2.19um1 x86_64 Ubuntu 12.04.1 LTS 
auth_cache_size = 20 M
auth_cache_ttl = 1 days
auth_debug = yes
auth_master_user_separator = *
auth_verbose = yes
default_process_limit = 1024
disable_plaintext_auth = no
log_timestamp = %Y-%m-%d %H:%M:%S
login_trusted_networks = 155.54.211.176/28
mail_debug = yes
mail_location = maildir:~/Maildir:INDEX=/var/indexes/%n
mail_privileged_group = mail
mdbox_rotate_size = 20 M
namespace {
  inbox = yes
  location = 
  prefix = 
  separator = .
}
namespace {
  hidden = yes
  list = no
  location = maildir:~/Maildir/expunged
  prefix = BORRADOS.
  separator = .
}
passdb {
  args = /etc/dovecot/master-users
  driver = passwd-file
  master = yes
  pass = yes
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
passdb {
  args = session=yes dovecot
  driver = pam
}
plugin {
  lazy_expunge = BORRADOS.
  quota = maildir:User quota
  quota_rule = *:storage=20G
  quota_rule2 = Trash:storage=+1G
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
  sieve_extensions = +imapflags
  sieve_max_redirects = 15
  zlib_save = gz
  zlib_save_level = 6
}
postmaster_address = postmaster at um.es
service anvil {
  client_limit = 2051
}
service auth {
  client_limit = 3072
  unix_listener auth-userdb {
    mode = 0666
  }
}
service doveadm {
  inet_listener {
    port = 24245
  }
}
service imap {
  process_limit = 5120
  process_min_avail = 6
  vsz_limit = 512 M
}
service ipc {
  unix_listener ipc {
    user = dovecot
  }
}
service lmtp {
  inet_listener lmtp {
    port = 24
  }
  process_min_avail = 10
  vsz_limit = 512 M
}
service pop3 {
  process_min_avail = 6
}
ssl = no
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
  args = username_format=%Ln /etc/dovecot/usersdb.%s
  driver = passwd-file
}
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocol lda {
  mail_plugins = " sieve"
}
protocol lmtp {
  mail_plugins = " sieve"
}
protocol pop3 {
  pop3_logout_format = top=%t/%p, retr=%r/%b, del=%d/%m, in=%i, out=%o
}
local 155.54.211.160/27/27 {
  doveadm_password = <password>
}