O+P Insights: Linux HW RAID Howto

Posted on January 03, 2008 in How-to

O+P Insights: Linux HW RAID Howto
Hardware RAID boxes are cool things. Plug them in and they behave like a big and fast disk. If properly configured, they'll be another 30% faster.
Issue

There is great software RAID support in Linux these days. I still prefer having RAID done by some HW component that operates independently of the OS. This reduces dependencies a great deal and takes load of the server.

Currently, my favorite hardware RAID configuration is rack-mountable servers with lots of disk bays, an 8 or 16 port Areca controller, and all configured as a large RAID 6 device.

So far, so simple, even without any optimization, performance is quite convincing. But surely there must be some things you can do to improve on that.

Raid Structure

RAID5 and RAID6 work by striping the data across multiple disks and writing parity information such that the data can be recovered when a disk breaks. This means, that even when writing a small amount of data, the parity information has to be updated for every write. Small updates are therefore not very effective on a RAID5/6 configuration. The optimal amount of data to be written to the system in one go is defined by the 'stripe-size' of the RAID configuration.

By working with 'stripe-sized' chunks of data you can help the RAID to work to its best performance. Often the stripe-size is 64 KByte, this means that everything should be aligned to 64 KByte.