Managing RAID on Linux - Derek Vadala [19]
Case 5: Video on Demand
This scenario offers the same considerations as Case 1, the site serving images. RAID-1, with multiple member disks, offers great read performance. Since writes aren't very frequent when working with video on demand, the write performance hit is okay.
Disk Failures
Another benefit of RAID is its ability to handle disk failures without user intervention. Redundant arrays can not only remain running during a disk failure, but can also repair themselves if sufficient replacement hardware is available and was preconfigured when the array was created.
Degraded Mode
When an array member fails for any reason, the array is said to have gone into degraded mode. This means that the array is not performing optimally and redundancy has been compromised. Degraded mode therefore applies only to arrays that have redundant capabilities. A RAID-0, for example, has only two states: operational and failed. This interim state, available to redundant arrays, allows the array to continue operating until an administrator can resolve the problem—usually by replacing a failed disk.
Hot-Spares
As I mentioned earlier, some RAID levels can replace a failed drive with a new drive without user intervention. This functionality, known as hot-spares, is built into every hardware RAID controller and standalone array. It is also part of the Linux kernel. If you have hardware that supports hot-spares, then you can identify some extra disks to act as spares when a drive failure occurs. Once an array experiences a disk failure, and consequently enters into degraded mode, a hot-spare can automatically be introduced into the array. This makes the job of the administrator much easier, because the array immediately resumes normal operation, allowing the administrator to replace failed drives when convenient. In addition, having hot-spares decreases the chance that a second drive will fail and cause data loss.
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Warning
Hot-spares can be used only with arrays that support redundancy: mirrors, RAID-4, and RAID-5. Striped and linear mode arrays do not support this feature.
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Hot-Swap
All of the RAID levels that support redundancy are also capable of hot-swap. Hot-swap is the ability to removed a failed drive from a running system so that it can be replaced with a new working drive. This means drive replacement can occur without a reboot. Hot-swap is useful in two situations. First, you might not have enough space in your cases to support extra disks for the hot-spare feature. So when a disk failure occurs, you may want to immediately replace the failed drive in order to bring the array out of degraded mode and begin reconstruction. Second, although you might have hot-spares in a system, it is useful to replace the failed disk with a new hot-spare in anticipation of future failures.
Replacing a drive in a running system should not be attempted on a conventional system. While hot-swap is inherently supported by RAID, you need special hardware that supports it. This technology was originally available only to SCSI users through specially made hard drives and cases. However, some companies now make hot-swap ATA enclosures, as well as modules that allow you to safely hot-swap normal SCSI drives. For more information about hot-swap, see the Cases, Cables, and Connectors section, later in this chapter, and the Managing Disk Failures section in Chapter 7.
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Warning
Although many people have successfully disconnected traditional drives from running systems, it is not a recommended practice. Do this at your own risk. You could wipe your array or electrocute yourself.
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Hardware Considerations
Whether you choose to use kernel-based software RAID or buy a specialized RAID controller, there are some important decisions to make when buying components. Even if you plan to use software RAID, you will still need to purchase hard drives and disk controllers. The first step is to determine