ZyXEL Communications NSA221 User Manual page 158

Chapter 7 Storage
• Mirroring
In a RAID system using mirroring, all data in the system is written
simultaneously to two hard disks instead of one. This provides 100% data
redundancy as if one disk fails the other has the duplicated data. Mirroring
setups always require an even number of drives.
• Duplexing
Like in mirroring, all data is duplicated onto two distinct physical hard drives but
in addition it also duplicates the hardware that controls the two hard drives (one
of the drives would be connected to one adapter and the other to a second
adapter).
• Striping
Striping is the breaking up of data and storing different data pieces on each of
the drives in an array. This allows faster reading and writing as it can be done
simultaneously across disks. Striping can be done at the byte level, or in blocks.
Byte-level striping means that the first byte of the file is sent to the first drive,
then the second to the second drive, and so on. Block-level striping means that
each file is split into blocks of a certain size and those are distributed to the
various drives. The size of the blocks used is also called the stripe size (or block
size).
• Parity
In mirroring 50% of the drives in the array are reserved for duplicate data.
Parity is another way to allow data recovery in the event of disk failure using
calculations rather than duplicating the data. If you have 'n' pieces of data,
parity computes an extra piece of data. The'n+1' pieces of data are stored on
'n+1'drives. If you lose any one of the 'n+1' pieces of data, you can recreate it
from the 'n' that remain, regardless of which piece is lost. Parity protection is
used with striping, and the "n" pieces of data are typically the blocks or bytes
distributed across the drives in the array. The parity information can either be
stored on a separate, dedicated drive, or be mixed with the data across all the
drives in the array.
Note: In the following figures, A1, A2, A3 and so on are blocks of data from the A file.
Similarly, B1, B2, B3 and C1, C2, C3 ar blocks of data from the B and C files.
JBOD
JBOD allows you to combine multiple physical disk drives into a single virtual one,
so they appear as a single large disk. JBOD can be used to turn multiple different-
sized drives into one big drive. For example, JBOD could convert 80 GB and 100
GB drives into one large logical drive of 180 GB. If you have two JBOD volumes
(with one disk in each), a failure of one disk (volume) should not affect the other
volume (disk). JBOD read performance is not as good as RAID as only one disk
can be read at a time and they must be read sequentially. The following figure
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NSA221 User's Guide