[Dovecot] dsync backup gets stuck... fails

[Jeff Gustafson]

On Sat, 2012-08-11 at 00:56 +0200, Daniel Parthey wrote:
> Hi Jeff,
> Please start by following the instructions at
> http://dovecot.org/bugreport.html
> and post your 'doveconf -n' output in order to provide
> possibly important information about your system and configs.

Storage is local hardware-based RAID (SATA drives). FS is ext4.
No core dump that I know of.
Command used (mailbox is over 2GB):

# su vmail -c "dsync -u user at domain.com backup ssh vmail at bk05 dsync -c /etc/dovecot/dovecot.conf -u user at domain.com"

Config file:	
# 2.1.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-220.17.1.el6.x86_64 x86_64 CentOS release 6.2
(Final) 
auth_mechanisms = plain login
default_client_limit = 15000
default_process_limit = 10000
disable_plaintext_auth = no
listen = *
mail_gid = vmail
mail_location = mdbox:~/mdbox
mail_plugins = zlib quota stats
mail_uid = vmail
mmap_disable = yes
namespace {
  inbox = yes
  location = 
  prefix = INBOX.
  separator = .
}
passdb {
  args = /etc/dovecot/conf.d/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  quota = dict:User noenforcing quota::file:%h/dovecot-quota
  stats_refresh = 30 secs
  stats_track_cmds = yes
  zlib_save = gz
}
protocols = imap pop3
service auth {
  client_limit = 10000
  unix_listener auth-userdb {
    mode = 0666
  }
}
service imap-postlogin {
  executable = script-login /usr/bin/postlogin-imap.sh
  user = $default_internal_user
}
service imap {
  drop_priv_before_exec = yes
  executable = imap
  process_limit = 10000
}
service pop3-postlogin {
  executable = script-login /usr/bin/postlogin-pop.sh
  user = $default_internal_user
}
service pop3 {
  drop_priv_before_exec = yes
  executable = pop3
  process_limit = 2500
}
service stats {
  fifo_listener stats-mail {
    mode = 0600
    user = vmail
  }
}
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
userdb {
  driver = prefetch
}
userdb {
  args = /etc/dovecot/conf.d/dovecot-sql.conf.ext
  driver = sql
}
protocol lmtp {
  mail_plugins = zlib quota stats
}
protocol lda {
  mail_plugins = zlib quota stats
}
protocol imap {
  mail_max_userip_connections = 100
  mail_plugins = zlib quota stats imap_quota imap_stats
}
protocol pop3 {
  mail_max_userip_connections = 30
  mail_plugins = zlib quota stats
}