RAID 10 naturally only works with an even number of disks and always has a usable capacity of 50%. RAID 10 allows a total of 2 disk failures, one per mirrored set. What is RAID 10? RAID 10 consists of two or more sets of mirrored disks striped together. Let's start by looking at the diagram for RAID 10 (1+0). These RAID levels combine the previously discussed RAID 1, 5, and 6 with striping (RAID 0) at a higher level. There is no similar performance gain for writing, as the RAID controller writes each block to a single disk at a time and calculates the parity data for each complete stripe. In terms of performance, a RAID 5 array's maximum read speed is a factor of the number of disks minus one, given that there is effectively one parity drive. During the rebuilding operation, the array will be vulnerable to another disk failure, which would mean a total loss of data. It does not scale well with today's enormous hard drives and could take hours, if not days to complete. Rebuilding reads all the data from the remaining disks, calculates the lost data, and writes the recovered data back to disk. Rebuilding a RAID array takes time and a significant amount of processing power. That means A2 can be computed using A1, A2 and A p.
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Its contents are such that all the data blocks plus the parity block equal zero when XOR'ed together. It can be recovered in a process called rebuilding, where the RAID array is fixed using the parity block. Let's say that disk 1 fails, meaning that A2 is lost. So how does parity help with fault tolerance? If we look at the diagram above, we see that the data labeled A is split into 3 parts plus a parity, A p. Therefore the minimum number of drives is 2 + 1 - two drives for the actual data and one for the parity. However, to make it more fault-tolerant than RAID 0, a particular block of data called a parity block is, effectively, written to an extra disk. Other RAID levels offer excellent reliability at less cost.ĭiagram showing RAID 5 configuration using 4 disks (3 disks is the minimum).Ī RAID 5 array is similar to RAID 0, as data is striped across several disks. However, the usable size would only be a third of the total capacity of the disks. If you had three mirrored drives, then two disks could fail without data loss. You might think to add more disks to a RAID 1 array, but this will have the effect of dramatically increasing the cost per usable capacity.
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You'll need to choose your RAID hardware and software carefully to maximize read performance. However, in practice, it is usually considerably slower. For reading, in theory, the maximum speed should be the sum of the speeds of the two disks. The write performance will be the same as writing to one disk since the RAID writes the same data to both disks simultaneously.
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The performance, however, is nowhere near as good as RAID 0. The data is mirrored on each drive, so even if one drive fails, you still have all your data on the other drive. In contrast to RAID 0, RAID 1 is all about fault tolerance and reliability. Just make sure there is a complete copy of the data somewhere else. Preferably to magnetic tape, but you could also use on-line remote backup or a second RAID array. Please note that whatever RAID configuration you choose, you still need to backup the data. These are known as " RAID levels", and we are going to explore what they are and their characteristics in the following sections. Now there are many ways to arrange and configure these inexpensive hard disks, depending on whether you want high performance (e.g., for video editing), high reliability, low cost or something in between. It allows the creation of a single logical disk (looks like one disk from the computer's point of view) made up of many cheap hard drives. RAID is an acronym that stands for Redundant Array of Inexpensive Disks, or, occasionally, Redundant Array of Independent Disks. What was the solution? Use many disks together, acting as one. Soon cheaper hard drives came along, but they were not very reliable at all, with failures being all too common. The thing is that when they failed (everything fails eventually), all of the data would be lost (unless you had a backup) and the expensive disk would need to be replaced. In the early days of computing, mainframes used large and expensive hard disks, designed to be highly reliable. If this is your first time configuring a RAID array, you might be unsure as to exactly what one is.