There are a number of factors that determine how to control RAID rebuild times and how long a disk drive rebuild time with RAID will take. These factors include what the RAID level is, the number of disks, the size of the disks, the type of disks, and where and how the RAID functionality is implemented. For example, RAID can be implemented in software in an operating system or file system. If RAID is implemented in a software system, then it relies on a primary processor, or offload assistance resources that compete with other applications via a PCIe RAID adapter card. PCIe RAID adapter cards also offload the functions that have hardware acceleration to boost rebuilds via controllers. Some RAID controllers provide better support for faster rebuilds than others, while others may take longer to rebuild the same disk drive.
Different RAID configurations may have an impact on rebuild time as well. For example, a mirror (RAID 1) should rebuild faster than a multi-drive RAID 4, 5 or 6. Other factors that may impact rebuild times include the size or capacity of the disk drive itself. As the data increases, the longer it will take to copy and rebuild. Also, the number of drives in a RAID group for some controllers and the I/O stripe size of data being spread across the disks may impact RAID rebuild times. If RAID rebuild times are a major concern for you, take a step back and rethink how you are configuring and deploying your storage along with the types of disks and storage solutions you purchase.
Be careful when you select your RAID disk drives and controllers, because some types of drives and controllers may be more susceptible to false rebuilds than others. Also, keep in mind that if your goal is to reduce your data storage costs by using more low cost, high-capacity disk drives and controllers, then expect to pay for the rebuild times. On other hand, investing in a solution that is not as likely to fail, will not result in false drive rebuilds, and can perform the rebuild faster on larger drives than others may be worth your time and investment.
This was first published in May 2010