Storage Terminology - ZyXEL Communications NSA-2401 User Manual

6.9.1 Storage Terminology

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.
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.
6.9.2 JBOD
• Minimum number of disks: 1
• Total capacity: Sum of the member disks
• Advantages: Maximum storage capacity, especially for disks of mixed sizes. Flexibility
(you can add disks to the JBOD
• Disadvantages: Not as fast or reliable as RAID.
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 several different-sized drives into one
big drive. For example, JBOD could convert 100 GB, 200 GB, 250 GB, and 500 GB drives
into one large logical drive of 1050 GB. Since data isn't striped across disks, if one disk fails,
NSA-2401 User's Guide
Chapter 6 Storage Screens
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