Question

RAID technology is introduced in the book, but, however, not in detail. Due to the internal configuration and implementation difference, RAID systems are categorized by level such as RAID-0, RAID-1, ...., RAID-10, or nested (hybrid) types, such as RAID 0+1 or RAID 5+1. Among all RAID systems, RAID 1 and RAID 5 are the most common RAID levels.

Here, in particular, RAID-5 is introduced. A RAID-5 system requires a minimum of three hard disks and no maximum as shown in the figure below.

Based on this 3-disk RAID-5 system, every input (e.g., a file) that needs to be stored is evenly divided and stored into two disks by a striping process. For example, the figure shows there are three files, pink, blue, and yellow, already evenly stripped and stored. To illustrate RAID-5 technology easily, we assume each simple digit represents half of the content of each file. When the pink file is stored in Disk 0 and 1, a "parity" is calculated by using a formula, say, an addition, which results 7+2=9 and is stored in Disk 2. Similarly, the parity of blue and yellow files are calculated to be 5 and 8, respectively. It's very important to notice that, in every group of three files, their parity are evenly distributed and stored in a different disk.

As seen in the above figure and description, because data are striped evenly across all of the disks, RAID-5 evenly balances reads and writes with no single disk being a bottleneck. Striping all parity evenly also allow data to be reconstructed in case of a disk failure. Therefore, RAID 5 is considered one of the most secure RAID configurations.

Here is the first question for you to solve. If a new file, say, containing 17 and 8, needs to store after the above three, how is it going to be stripped and what's its parity of this new file using the same formula?

Second question. After the fourth file is stored, if the entire Disk 0 fails or crashes now, then, "7" of the pink file, "4" of the blue file, the parity "8" of the yellow file, and some part of the 4th file all become inaccessible. If you represent the controller software of this RAID-5 system, describe how you (or the RAID controller in this case) would rebuild or reconstruct Disk 0 completely file by file.

Last question. If there are 1000 files stored in this 3-disk RAID-5 system and Disk 0 and Disk 2 are both crashed, then, is it possible for any files still accessible?

Answer 1

Explanation:

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