ZyXEL Communications NAS320 User Manual page 171

These are some terms that you need to know in order to understand storage systems.
• 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 are 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 shows disks in a single
JBOD volume. Data is not written across disks but written sequentially to each disk until it's full.
Table 28 JBOD
A1
A2
A3
A4
DISK 1
Media Server User's Guide
B1
B2
B3
B4
DISK 2
Chapter 8 Storage
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