Raid - ZyXEL Communications NSA-220 User Manual

You should not restart the NSA while the NSA is resychronizing a volume as
this will cause the synchronization to begin again after the NSA fully reboots.
You can access data on a RAID volume while it is resychronizing, but it is not
recommended.

6.4 RAID

This section contains theoretical background on JBOD and the RAID levels used on the NSA.
Skip to the next section if you already understand RAID and know what storage system you
want to use on the NSA.
Redundant Array of Independent Disks (RAID) is a method of storing data on multiple disks
to provide a combination of greater capacity, reliability, and/or speed. JBOD (Just a Bunch Of
Disks) is not a RAID storage method but it is included in this discussion.
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
NSA-220 User's Guide
Chapter 6 Storage Screens
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