HP Z840 Maintenance and Service Guide - Page 124
Software RAID solution, Software RAID considerations, Performance considerations
View all HP Z840 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 124 highlights
● Write Through:The configuration might result in slower performance. ● Always Write Back: The configuration results in optimal performance, but there is a risk of data loss in the event of power failure. ● Write Back with BBU: If you have installed a BBU, write back is enabled only when the battery has a sufficient charge. During a learning cycle, the caching policy reverts to write-through until the learning cycle is complete. 7. Select Yes to accept the warning, and then select Next. 8. Select Accept, and then select Yes to save the configuration. 9. Select Yes to initialize the virtual drive you created. Software RAID solution This section summarizes software RAID considerations that are specific to the Linux environment, and provides links to additional configuration resources. Software RAID considerations The Linux kernel software RAID driver (called md, for multiple device) offers integrated software RAID without the need for additional hardware disk controllers or kernel patches. Unlike most hardware RAID solutions, software RAID can be used with all types of disk technologies, including SATA, SAS, SCSI, and solid-state drives. This software solution requires only minimal setup of the disks themselves. However, when compared to hardware-based RAID, software RAID has disadvantages in managing the disks, breaking up data as necessary, and managing parity data. The processor must assume some extra loading: disk-intensive workloads result in roughly double the processor 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 processor performance are very well balanced and already near bottleneck levels, this additional processor 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 disk 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. 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 any bottleneck is in the system. Several RAID levels offer improved performance relative to stand-alone disk performance. If disk throughput is restricted because of a single disk controller, RAID can probably do little to improve performance until another controller is added. Conversely, if raw disk performance is the bottleneck, a tuned software RAID solution can dramatically improve the throughput. The slower disk performance 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. 114 Appendix B Configuring RAID devices