dovecot 2.3.7.2-1~bionic: Performance issues caused by excessive IO to ~/mdbox/mailboxes/INBOX/dbox-Mails/dovecot.index.tmp

[Ralf Hildebrandt]

I set up system copying all mails to a backup system.

This used to work without a hitch - now in the last few days mails
would pile up in the Postfix Queue, waiting to be delivered using the
lmtp transport into dovecot.

So dovecot was being slow, but why? After all, nothing changed.

After reading some articles on stackoverflow I found a way of finding
out which file gets the most IO:

% sysdig -c topfiles_bytes;

This command quickly pointed to ~/mdbox/mailboxes/INBOX/dbox-Mails/dovecot.index.tmp
That file was written excessively. 

I then put ~/mdbox/mailboxes/INBOX/dbox-Mails/ into tmpfs and alas, the queue would drain quickly.

But why is that? Why would the index file be updated so often?

This is dovecot 2.3.7.2-1~bionic

# 2.3.7.2 (3c910f64b): /etc/dovecot/dovecot.conf
# OS: Linux 5.0.0-29-generic x86_64 Ubuntu 18.04.3 LTS 
default_vsz_limit = 2 G
lmtp_user_concurrency_limit = 10000
mail_attachment_dir = /home/copymail/attachments
mail_attachment_hash = %{sha256}
mail_fsync = never
mail_location = mdbox:~/mdbox
mail_plugins = zlib fts fts_lucene
mdbox_rotate_size = 128 M
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = username_format=%u /etc/dovecot/passwd
  driver = passwd-file
}
plugin {
  fts = lucene
  fts_languages = de,en
  fts_lucene = whitespace_chars=@.
}
protocols = " imap lmtp"
service imap-login {
  inet_listener imap {
    address = 127.0.0.1
    port = 143
  }
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
service lmtp {
  inet_listener lmtp {
  address = 10.0.5.208
    port = 1025
  }
  process_min_avail = 5
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0660
    user = postfix
  }
}
ssl_ca = /etc/ssl/certs/ca-certificates.crt
ssl_cert = </etc/ssl/certs/mail-cbf.pem
ssl_cipher_list = EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH+aRSA+RC4:EECDH:EDH+aRSA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
ssl_prefer_server_ciphers = yes
userdb {
  args = username_format=%u /etc/dovecot/passwd
  driver = passwd-file
}
protocol lmtp {
  mail_plugins = zlib fts fts_lucene
}

-- 
Ralf Hildebrandt
  Geschäftsbereich IT | Abteilung Netzwerk
  Charité - Universitätsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  ralf.hildebrandt at charite.de | https://www.charite.de