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Because of RAID 5 stripes data and parity bits across all disks, it is very tolerant of single-disk failures, although this reduces the disk capacity slightly. If a disk fails, it simply has to be replaced and the system can go on. Further data reads are calculated from the parity so that end-users do not even notice the disk failure.
RAID 5 is similar to RAID 4, but the dedicated drive used for parity was removed and replaced with a distributed algorithm. This resolves the bottlenecks caused by a dedicated parity disk.
Advantages:
- Can tolerate the loss of a single drive
- Good random read performance
- Good sequential read and write performance
Disadvantages:
- Parity calculation may slow down the system
- Can use N-1 drive space.
- Due to parity overhead, random write performance takes a hit
Time is of the Essence
The fundamental problem is that it takes too long to fill the large disks we are now using just to regain redundancy. All of these RAID levels will use a single disk to replace a failed disk and all will need to fill the one disk with data. The time it takes to recover from the failed disk cannot be less than the size of the disk divided by its sequential write speed. For a 72GB disk with an 80MBps write rate we get 72,000MB / 80MBps = 900 seconds, about 15 minutes. This is an acceptable rebuild time and ten years ago when we used 72GB disks RAID was good. Today we use at least 1TB disks and their sequential write rate has only gone up to around 115MBps, the same math is 1,000,000MB / 115MBps = approximately 8700 seconds which is nearly two and a half hours. If you are using 4TB disks then your rebuild time will be at least ten hours.
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