Poor write performance on Fusion MPT SAS RAID controllers

If you have a hardware RAID controller using mptsas module on your Linux server, you may notice your write performance is extremely poor – i.e. some database workloads can be 30x slower than on other servers (i.e. using Linux software RAID).

An example card below – lspci shows:

How can you fix it? See below.

The issue happens because write caching is by default disabled on drives behind the RAID – and there is no way to enable it in controller firmware / BIOS, or by running scsi-config command.

To enable write caching, you may use below commands in your startup scripts:

That’s it! Performance should be back to normal. Of course you have to be aware of the dangers of write-caching and potential data loss on unexpected power outage.

Other, useful commands:

You can also add /dev/sg0 and /dev/sg1 to monitoring by smartmontools (/etc/smartd.conf).

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