Peer Heinlein writes:
At the moment we have about 30.000 Accounts with round about 150 GByte mailspace, located on a NFS-storage with 7.200 RPM RAID-1-SATA-discs. We have two Postfix- and two IMAP-Nodes.The performance on the discs are bad -- iostat shows up to 100% IO-usage.
The first thing to check is whether you're getting nailed by Gamin/FAM. Gamin/FAM is notoriously difficult to set up right with NFS. If you're using FAM, famd also needs to run on the NFS servers, for it to work right. If you're using Linux-gamin, certain Linux kernel versions just fall apart, under high load.
For sure -- it's possibe to build a better NFS-storage with 15.000 RPM SCSI -- but we have to scale up to 60.000 or 90.000 Accounts with 400 / 500 GByte Maildir-Data. But will that help?
Of course. The only question is whether the help will be sufficient.
Yes, it is possible to use an IMAP-proxy to spread the storage over several IMAP-Nodes with local storage. But that includes to many single point of failures und is very unflexible in the handling of performance peaks.
What single point of failure? Put two SCSI hard drives, in hot-swap bays, in each IMAP node, in a RAID-1 configuration. If one of the drive fails, hot-swap it with a spare. If the entire server fails, have an empty server on standby, with two empty hot-swap bays.
Two weeks ago, one drive in one of my servers remapped a couple of bad sectors but still got itself kicked out of the array. It was one of a pair of ordinary Cheetah drives, connected to a run-of-the-mill Adaptec host. I replaced it with a spare, and rebuilt all partitions, without any downtime.
Yes, if an entire server fails there will be some downtime as you move the disks on a standby. But how often does that happen, when you use high-quality server hardware? And if you do need nine-9s then yes, you will have to open up your wallet and pay for some very expensive hardware, Netapp, etc… You're not paying for performance, but for uptime. You only get good performance as part of the package.
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