Eranga Udesh schrieb am Wed, Dec 12, 2001 at 12:11:55PM +0600:
[...]
*
* I need to setup a clustering IMAP mail server. So I'm planning to use a NAS
* for storage) and couple of nodes with Linux OS. But when comes to File
* System, I found that Cyrus IMAP server has problems with NFS file system.
I'm no expert on NFS, but some hints anyway.
1. Cyrus and other POP3/IMAP4 mail storage systems utilise especially one
ressource very heavily: IO. CPU is mostly no real problem. So putting
the very bottleneck into the network is not very wise. Add to that the
fact that most NFS implementations are not very efficient/performant
respective to speed.
2. Your NAS will be a single point of failure.
I built an IMAP cluster some weeks ago that tried to address the sacling
problem somewhat different.
A. Each node has direct access to its storage via SCSI/Fiberchannel bus.
B. Each node has a backup system with the same access (the storage is
sort of a SAN and administered by the kimberlite clustering software).
The backup system will access the devices only on failover situations
and will then replace the original system.
C. Each node has therefore its own user space. As the Cyrus IMAP software
cannot talk to others over several nodes which user is where, we used
a separate Linux cluster at the front to proxy every incoming user
request to its respective Cyrus IMAP server. We used perdition as the
proxy and an LDAP server as the main user base. Every user within
the LDAP is marked to which node he belongs to.
Via POP3/IMAP4-Proxying and LDAP as the user base we are now able to
scale into a very high number of users without losing performance: Every
Nth number of users we can set up a new node with maximum IO performance.
Additionally, no single point of failure exists on server level, at net-
work level we avoided single point of failures by setting up a layer-4-
switch (take an L5 or LVS, whatever you need) which has a backup system
as well.
- Birger
