Understanding RAID

About RAID

Appliances (with the exception of the C1100 and C1105 systems) use RAID Level 4 to ensure data integrity even when one of the disks fails. The file system design, together with the support for RAID, optimizes appliance performance and enables you to expand the appliance's disk storage capacity incrementally.

The RAID disk structure on your appliance is factory-installed; you do not need to configure the storage system before using the appliance. However, when planning to optimize disk space or when replacing or adding disks, you must be familiar with how the appliance uses its disk array.

In the RAID file system, different disks have different functions. Most of the disks are data disks. One disk in the array is the parity disk, which enables the appliance to recover the data on a data disk if one fails. You can also install one or more disks to be used as hot spares in the event of disk failure.

Data disks

Data disks are those on which cached objects, logs, and other data are stored.

Parity disks

After a data disk failure, the appliance uses the parity disk in conjunction with the other data disks to reconstruct the failed disk's data and write it to a hot spare disk. The parity disk must be at least as large as the largest data disk. For each 28 disks added to an appliance, one drive is used for parity.

For more information about how the appliance recovers from disk failures by using the parity disk or hot spare disk, see Handling disk failure.

Hot spare disks

In addition to data and parity disks, you can install one or more hot spare disks in the appliance. A hot spare disk is not part of the RAID disk array and does not contain cached objects. After a disk failure, the appliance automatically rebuilds data onto a hot spare disk, which then replaces the failed disk in the RAID array. Using a hot spare disk avoids a system shutdown and returns the system to full performance.

A hot spare disk cannot replace a failed disk that is larger than itself. If you use only one hot spare disk in an appliance, the hot spare disk must be as large as the largest file system disk. If you have multiple hot spare disks installed, the system uses the smallest hot spare disk needed to replace the failed disk.

Hot swapping disks

Some models of the appliance allow you to hot swap disks. Hot swapping a disk means installing or removing it from the disk shelf while the appliance is running with minimal interruption to the service. For example, you might want to hot swap a disk into an appliance to replace a disk or to add a hot spare disk.