dovecot - Nov 2011 - Dovecot performance issues with many writes

We are currently experiencing performance issue with our Dovecot system
which we believe is caused by excessive writes to the dovecot files. The
confusing thing is that we are seeing more writes than reads on our Dovecot
volume when you would assume that most of the IO should be reads from
customers checking their mail.

We're seeing reads vs. writes similar to the following:

# iostat -d 5 -x
Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz
avgqu-sz   await  svctm  %util
dm-4              0.00     0.00  485.80  865.80  3886.40  6926.40
8.00    28.69   19.63   0.70  94.00
drbd0             0.00     0.00  485.80  865.80  3886.40  6926.40
8.00   111.42   94.44   0.74  99.60

We can't figure out why we would be seeing twice as many writes as we are
reads. Could this have something to do with the index files?

Appreciate any assistance on this issue.

On 17.11.2011, at 23.30, Andy Robbins wrote:
> We can't figure out why we would be seeing twice as many writes as we
are
> reads. Could this have something to do with the index files?
dovecot -n output would be helpful.

# dovecot -n
# 1.2.9: //etc/dovecot.conf
# OS: Linux 2.6.18-164.9.1.el5 x86_64 CentOS release 5.4 (Final)
base_dir: /var/run/dovecot/
log_path: /var/log/dovecot.log
protocols: imap pop3
ssl_cert_file: /etc/postfix/ssl/smtp1.domain.com.cert
ssl_key_file: /etc/postfix/ssl/smtp1.domain.com.key
disable_plaintext_auth: no
login_dir: /var/run/dovecot//login
login_executable(default): //libexec/dovecot/imap-login
login_executable(imap): //libexec/dovecot/imap-login
login_executable(pop3): //libexec/dovecot/pop3-login
login_user: postfix
login_greeting: Welcome
login_process_per_connection: no
login_processes_count: 2
mail_max_userip_connections: 50
mail_uid: 500
mail_gid: 500
mail_location: maildir:~/Maildir
mail_debug: yes
mmap_disable: yes
mail_nfs_storage: yes
mail_nfs_index: yes
mail_executable(default): /etc/dovecot/last-login-imap.sh
mail_executable(imap): /etc/dovecot/last-login-imap.sh
mail_executable(pop3): /etc/dovecot/last-login-pop3.sh
mail_plugins: quota
mail_plugin_dir(default): //lib/dovecot/imap
mail_plugin_dir(imap): //lib/dovecot/imap
mail_plugin_dir(pop3): //lib/dovecot/pop3
pop3_uidl_format: %f
lda:
  postmaster_address: postmaster at domain.com
  auth_socket_path: /var/run/dovecot/auth-master
  log_path: /var/log/dovecot-deliver.log
  info_log_path: /var/log/dovecot-deliver.log
  fsync_disable: no
auth default:
  mechanisms: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 APOP
  user: vmail
  username_translation: %@
  verbose: yes
  passdb:
    driver: ldap
    args: /etc/dovecot/dovecot-ldap.conf
  userdb:
    driver: ldap
    args: /etc/dovecot/dovecot-ldap.conf
  socket:
    type: listen
    client:
      path: /var/spool/postfix/private/auth
      mode: 432
      user: postfix
      group: postfix
    master:
      path: /var/run/dovecot/auth-master
      mode: 384
      user: vmail
      group: vmail
plugin:
  quota: maildir:User quota
  quota_rule: *:storage=0

On Thu, Nov 17, 2011 at 03:30:07PM -0600, Andy Robbins
wrote:
> We are currently experiencing performance issue with our Dovecot system
> which we believe is caused by excessive writes to the dovecot files. The
> confusing thing is that we are seeing more writes than reads on our Dovecot
> volume when you would assume that most of the IO should be reads from
> customers checking their mail.
Have you mounted the fs without noatime options? 


  -jf

Am Donnerstag, 17. November 2011, 22:30:07 schrieb Andy Robbins:
> We are currently experiencing performance issue with our Dovecot
> system which we believe is caused by excessive writes to the dovecot
> files. The confusing thing is that we are seeing more writes than
> reads on our Dovecot volume when you would assume that most of the
> IO should be reads from customers checking their mail.
Yes, but your read-requests are answered from your local kernel-buffer / 
cache and aren't really read from your hard-disk.

(At least if you have enough free memory for a cache, that's large 
enought.)

Incoming mails ARE written on the disk, but read later on just from your 
cache.

Peer



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