HP xw4400 Workstation - Manual (Page 23)

5 Software RAID in HP Linux
Workstations
Introduction
This chapter provides a summary of software RAID considerations for HP workstations running the Linux
operating system. This chapter also provides references to procedures on configuring software RAID.
NOTE: While BIOS messages might refer to hardware RAID, only SAS hardware RAID is supported
by HP Linux Workstations. See Installing and configuring SAS hardware RAID on page 21 the for
information about configuring hardware RAID.
Software RAID considerations
The Linux kernel offers integrated software RAID without the need for additional hardware disk
controllers or kernel patches. All that is required are multiple hard disks and a small amount of setup.
Unlike most hardware RAID solutions, software RAID can be used with all types of disk technologies,
including SATA, SAS, SCSI, and solid state drives.
Compared to hardware-based RAID, software RAID has disadvantages in managing the disks,
breaking up data as necessary, and managing parity data. The CPU must assume some extra loading.
It has been found that heavily disk-intensive workloads result in roughly double the CPU overhead (for
example, from 15% to 30%) when software RAID is in use. For most applications, this overhead is
easily handled by excess headroom in the processors. But for some applications where disk and CPU
performance are very well balanced and already near-bottleneck levels, this additional CPU overhead
can become troublesome. Hardware RAID offers advantages because of its large hardware cache and
the capability for better scheduling of operations in parallel. However, software RAID offers more
flexibility for disk and disk controller setup. Additionally, hardware RAID requires that a failed RAID
controller must be replaced with an identical model to avoid data loss, whereas software RAID imposes
no such requirements.
Some software RAID schemes offer data protection through mirroring (copying the data to multiple disks
in case one fails) or parity data (checksums that allow error detection and limited rebuilding of data in
case of a failure), but all software RAID solutions on HP workstations require the shutdown of the system
so that the failed drive can be replaced before redundancy can be restored. The replacement of failed
drives in software RAID requires only a minimum amount of work.
Performance considerations
Disk I/O bandwidth is typically limited by the system bus speeds, the disk controller, and the disks
themselves. The balance of these hardware limitations, as affected by the software configuration,
determines where the real bottleneck is in the system.
Introduction 19