Software Raid Solution; Software Raid Considerations; Performance Considerations; Configuring Software Raid - HP Z1 Maintenance And Service Manual

Software RAID solution

This section provides a summary of software RAID considerations when running the Linux operating
system, as well as references to configuration procedures.

Software RAID considerations

The Linux kernel software RAID driver (called
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: disk-intensive workloads result in roughly double the CPU overhead (for example, from 15%
to 30%). 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). For all software RAID solutions on HP workstations, redundancy can be
restored only after the system is shut down so that the failed drive can be replaced. In software RAID,
this replacement 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 the location of the real bottleneck in the system.
Several RAID levels offer improved performance relative to a standalone disk. If the disk throughput
is lessened by a single disk controller, there is probably little you can do with RAID to improve the
performance without adding another controller. On the other hand, if the raw disk performance is the
bottleneck, a tuned software RAID solution can dramatically improve the throughput. The slower the
disk is relative to the rest of the system, the better RAID performance will scale, because the slowest
piece of the performance pipeline is being directly addressed by moving to RAID.

Configuring software RAID

See the following sites for additional information about configuring software RAID on Red Hat
Enterprise Linux (RHEL) or SUSE Linux Enterprise Desktop (SLED):
Red Hat Enterprise Linux 6 — See the
●
docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/index.html
Red Hat Enterprise Linux 5 — See the
●
Red_Hat_Enterprise_Linux/5/html/Deployment_Guide/ch-raid.html
SLED 11 — See the
●
book_sle_deployment/?page=/documentation/sled11/book_sle_deployment/data/
book_sle_deployment.html
100 Appendix B Configuring RAID devices
md
Storage Administration Guide
Deployment Guide
Deployment Guide
at http://www.suse.com/documentation/sled11/
multiple device
, for ) offers integrated software RAID
at http://docs.redhat.com/docs/en-US/
at http://docs.redhat.com/