[Dovecot] NFS not responding generates authantication crash

[bart]

I am facing dovecot authentication problems caused by unresponding NFS
server. If there is even short break in communication with NFS server
keeping maildirs, the dovecot generates the avalanche of processes
(dovecot/imap and dovecot/pop3). The real number of connections was about 50
and after the problems occurs it rises to 1000. After about 3 hours the
limit of connections is filled up:

 dovecot: master: Warning: service(auth): client_limit (1000) reached,
client connections are being dropped

and next:

imap-login: Warning: Auth process not responding, delayed sending greeting
pop3-login: Warning: Error sending handshake to auth server: Broken pipe
imap-login: Warning: Error sending handshake to auth server: Broken pipe


When the mail storage was local there was no such problem. After moving to
NFS storage the problem arised when trying to make backup with dump (we
switched to rsync then) or now if NFS is not responding for a while. 

My dovecot conf for NFS is in accordane with: http://wiki2.dovecot.org/NFS

mmap_disable = yes
dotlock_use_excl = no # only needed with NFSv2, NFSv3+ supports O_EXCL and
it's faster
mail_fsync = always
mail_nfs_storage = no
mail_nfs_index = no

NFS shares are connected with no special parameters.

Can something be done on dovecot configuration to prevent from generating
those dovecot connections processes?
Or is it the problem with NFS configuration?



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