Dell Compellent Series 40 - Manual (Page 111)

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RAID 0
Data is striped across the available disks providing improved performance. RAID 0 does
not provide any data redundancy.
RAID 5
Maintains a logical copy of the data using a mathematically derived rotating parity stripe.
The parity stripe is derived from the data stripes. This method has less overhead for the
redundant information than RAID 10; however, write performance is slower than RAID 10
due to the calculation of the parity stripe for every write. Read performance is similar to
RAID 10.
RAID 6
Has striped data with dual distributed parity and provides fault tolerance from two drive
failures. The array can continue to operate with up to two failed drives. This makes larger
RAID groups more practical, especially for high availability systems. This becomes
increasingly important because large-capacity drives lengthen the time needed to recover
from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until
the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity
gives time to rebuild the array without the data being at risk if a (single) additional drive fails
before the rebuild is complete.
RAID 10
Data is striped across available disk drives and mirrored, providing data availability and
improved performance. Maintains a minimum of one full copy of all data on the volume.
RAID 10 provides optimum Read / Write performance, increased probability of withstanding
multiple failures, and the quickest restoration of data.
redundancy
Fault tolerance such that if some disks fail, all or part of the data stored by the array is not
lost. The cost of providing this feature is most typically increased disk space; RAID 10
requires a duplication of the entire data set. RAID 5-5 and 5-9 contain an error-correcting
algorithm stored on the array.
Remote Instant Replay
Point-in-time copy of a volume over a distance to a remote site. The sites can be active-
active, with bidirectional remote copies that can either have matched or split intervals.
Replay
Fully usable copy of data that contains an image of the data as it appeared at the point in
time at which the copy was initiated.
Replay space
Amount of space that is not actively being used by a volume and is contained in its replays.
This number does not include RAID overhead.