Cyrus HA & LB cluster summary

[Michael Menge]

Hi,

the Cyrus (HA and LB) Cluster question seems to popup very often the  
last months. With this post I try to summarize the info of the posts  
of the
following threads:
- Cyrus IMAP and MySQL mailboxes (Building load-balancing  cluster)
- NFSv4, anyone?
- Cyrus, clusters, GFS - HA yet again
- High availability email server.
- Cyrus & Lustre
I found some older threads which might be of intresst, but not upto  
date anymore:
- Cyrus 2.3 on shared filesystems
- Playing with replicated murder
- Using a SAN/GPFS with cyrus
- high-availability again
- Two Cyrus servers
- Cyrus, NFS and mail spools
- multiple cyruses via SAN
- Cyrus/NFS/SMB
- No NFS? Ok, how about GFS/GPFS

If some points are incorrect please feel free to correct them.


These are the setups for Cyrus in a HA and/or LB Cluster

- DNS / Perdition load balancing
   The users are split on several Cyrus server and DNS or Perdition is used to
   direct the user to the correct server. This setup is only a static  
LB cluster
   and it is not possible to share mailboxes between users on  
different servers.
   But this setup is very easy and there are no problems because there are no
   shared filesystems. In case one server crashes the some of the users will
   have no access to the mails. Mails may be lost depending on the kind of
   crash, the storage and the backup.

- Murder setup
   In this setup the users are split on several backends that are connected via
   frontendservers. This setup allows shared folders between users on different
   backends. This is like the DNS / Perdition setup only a LB cluster,  
but users
   can be moved easy from one backendserver to another. In case of crash this
   setup does not differ from the setup above.

- Replication
   Replication can be add to the two setups above. But you need either
   additional servers or you have to take care to set up replication
   server and client on one Cyrus server/backend.
   Replication is only in Cyrus 2.3.x and this setup needs manual intervention
   i case of failure. In this case the MUPDATE master / DNS / Perdition has to
   be set to point to the new server. With scripts it should be possible to do
   this automatical. The replication is asynchron so you might lose some mails.

- Shared filesystem Active/Passive
   This setup can add HA to a normal Cyrus server by storing the Mails and
   databases on a shared filesystem and monitoring the server with heartbeat.
   This setup should work with every shared filesystem and there should be no
   problems with bdb

- Shared filesystem Active/Active
   In this setup all users are on all server. The mailboxes and databases
   are on a shared filesystem so that changes on one server are visible on all
   other servers. Cyrus depends on the file-locking of the filesystem. NFSv4,
   GFS, Lustre, and some other shared filesystem affirm that they have
   file-locking across cluster nodes.
   The sockets, lock- and pid-files have to stay on a local
   filesystem or have to be made unique across cluster nodes.
   BDB seems to have problems in this setup, because the changes of the lock
   in the mmaped files are not instantly on all clients/nodes and the use of
   shared memory. You have to compile Cyrus without bdb-support to get rid of
   the errors. This setup needs no extra servers, and all use the same
   configuration. In case of server crash the user use one of the  
other servers.
   The server can be replaced with an clone of one of the other servers.
   The storage should be too on a HA  cluster to make the whole mailsystem ha.

The Active/Active Shared filesystem setup is discussed controvers.
This setup is not widely used and therefor not tested that mutch.
But Dave McMurtrie and Scott Adkins have reported of sucsessfull installations


The following shared Filesystems seem support the file-locking

GFS            http://sources.redhat.com/cluster/gfs/
Lustre         http://www.lustre.org/
NFSv4          http://www.nfsv4.org/
Polyserve CFS  http://www.polyserve.com
Veritas CFS  
http://www.symantec.com/enterprise/products/overview.jsp?pcid=1020&pvid=209_1




--------------------------------------------------------------------------------
M.Menge                                 Tel.: (49) 7071/29-70316
Universitaet Tuebingen                  Fax.: (49) 7071/29-5912
Zentrum fuer Datenverarbeitung          mail:  
michael.menge at zdv.uni-tuebingen.de
Waechterstrasse 76
72074 Tuebingen
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