Optimum RAID

Creating a software RAID using the Linux kernel is becoming easier and easier. With a call to mdadm and pvcreate, you can be well on your way to using LVM on top of a RAID 5 or RAID 10 device. In fact, the procedure for setting up a RAID system has gotten so simple that many users routinely click through the commands without too much consideration for how the various settings might affect performance.

When it comes to RAID, however, the default settings aren't always a prescription for optimum performance. As you will learn in this article, tailoring your filesystem parameters to the details of the RAID system can improve performance by up to 20%.

Lining up the RAID

Filesystems are deceptively complex pieces of software. At first, a filesystem might appear rather simple: Save a file and make sure you can get it back again. Unfortunately, even writing a small file presents issues, because the system caches data in many places along the way to the disk platter. The kernel maintains caches, your disk controller might have a small memory cache, and the drive itself has many megabytes of volatile cache. So, when a filesystem wants to make sure 100KB is really on the disk, a fairly complex and often slow operation takes place.

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