Modern CCTV systems require lots of storage so we often get asked a number of questions about different RAIDs; which is best, how to manage them and how to silence the infernal alarm beeps.
When selecting the right RAID for your system cost and redundancy is always important. Of course most people would like the safest system possible but adding hard drives which hopefully won’t be used and adding extra costs to a project isn’t desired.
Mirasys VMS supports a multi disk recording to minimise data loss and maximise recording continuity. At Mirasys UK we tend to use 3 types of RAID, each scheme provides a different balance between the key goals of reliability, availability and performance.
RAID 1 – Creates an exact copy or mirror of a disk. We use this on our system to create a copy of the OS disk so if it fails the copy kicks in and the system continues to work.
This array layout is used when data reliability and read performance are the most important factors.
RAID 5 – An example of this is 5 disks with 1 spare, when 1 of the 5 fails the spare is used and the failed disk is rebuilt onto it so no data is lost. RAID 5 stripes data across the disks and uses parity(p) data spread over the 5 disks which is used to recreate any of the disks that fails. However, when a disk fails a rebuild is needed to recover the lost data, during this rebuild increased stress is placed onto each of the disks increasing the chance of disk failure during the rebuild process.
RAID 6 – Similar to RAID 5 but allows for 2 disks to fail rather than 1. By allowing more disks to fail your data is safer and you can add more spare disks to do the work should they fail. RAID 6 places less stress on the disks during a rebuild by using 2 sets of parity data (p & q) rather than just p which greatly decreases the chance of disk failure during a rebuild. Recommended for high-demand storage applications, such as video storage.
So far the RAID systems described have been at BIOS level, we use LSI Raid cards which load before Windows, this means Windows see a single drive and is unaware of the RAID. But RAID concepts can occur at Windows level. Most common is RAID 0 spanned over multiple disks. This can result in greater read and write speed to the drive but creates the issue that if 1 of the disks fails the data is lost on them all as RAID 0 offers no data redundancy. When writing to a 1 disk drive with 80mb/per second read write you are limited to that 80mb per second but if you have 2 disks each with 80mb per second then you have a possible 160mb per second. With CCTV systems recording and faster frame rates and in higher resolutions than ever before this increase in potential throughput can be of great use. But it comes with risks, if one of the disks was to fail you lose the data on both disks.