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Consists of block-level striping with dedicated parity. This level was previously used by
NetApp, but has now been largely replaced by a proprietary implementation of RAID 4
with two parity disks, called RAID-DP.
Consists of block-level striping with information distributed among the drives. Upon failure
of a single drive, subsequent reads can be calculated from the distributed parity such that
no data is lost. RAID 5 requires at least three disks. Rebuilding an array requires reading
all data from all disks, opening a chance for a second drive failure and the loss of entire
Consists of block-level striping with double distributed parity. Double parity provides fault
tolerance up to two failed drives. This makes larger RAID groups more practical, especially
for high-availability systems, as large-capacity drives take longer to restore. RAID 6
requires a minimum of four disks.. The larger the drive capacities and the larger the array
size, the more important it becomes to choose RAID 6 instead of RAID 5 RAID 10 also
minimizes these problems.
Table 6-8: Redundant Array of Independent Disks (RAID) Levels
If an entire disk array or an entire NVR fails, then a redundant NVR should immediately take over the
responsibility for recording those cameras. When the primary NVR is brought back to life, the redundant
one should hand back control. When the operator reviews video recordings it should be transparent whether
the video was stored on the primary or redundant NVR, or indeed partially on both. It is possible to have
one redundant NVR act as a standby for several primary NVRs, which offers a more economical solution
instead of one redundant NVR per primary NVR. A ratio of 10-1 or 5-1 is common and depends on the
attitude toward risk. If 2 NVRs fail at the same time, the second NVR will not have a redundant NVR for
Most video management system (VMS) architectures include some kind of central server that provides
system-level monitoring, manages users and alarms, system integration, NVR failover commands and many
If the network fails, then the consequences depend on the location of the failure. If it is between the edge
device and the NVR, then nothing will be recorded. A solution to this is to consider some local storage on
the edge device. However, the edge device is usually fairly exposed to the public and so this means recorded
video is somewhat vulnerable, especially if it is a simple memory card that can be popped out. If the network
fails between the NVR and central server, then the NVR should be designed to continue recording, as this
is essentially the same failure scenario as the central server failing.
The most common kinds of NVR storage are SCSI (directly connected via SCSI cables to the NVR), or the
two kinds of more scalable networked storage: Network Attached Storage (NAS) or Storage Area Networks
NAS and SAN both have their advantages and disadvantages. A NAS connects directly to the network
using TCP/IP over Ethernet CAT 5/6 cabling and is assigned an IP address just like any other network
device. In most cases, no changes to the existing network infrastructure need to be made in order to install
a NAS solution.
A SAN is a dedicated storage area network that is interconnected through a fiber channel protocol using
either 1-gigabit or 2-gigabit fiber channel switches or fiber channel host bus adaptors. Devices such as file
servers connect directly to the storage area network through the fiber channel protocol. Where a NAS uses
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