Minimum Disks for Mixed RAID 1 and RAID 5 Configurations

An application needs 10GB of RAID 1 for log files, 20GB of RAID 5 for data files, and 20GB of RAID 5 for the operating system. All disks will be 10GB in capacity. Which of the following is the minimum number of disks needed for this application?

6
7
8 Source Reference Answer
9

Community Votes

100%

100% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

The exam tests your understanding that RAID configurations must be calculated independently per array; you cannot combine disparate RAID needs into a single virtual pool unless specified.

This question tests the ability to calculate raw disk requirements for specific RAID levels (RAID 1 and RAID 5) to meet usable capacity goals. The community consensus confirms that separate arrays are required, leading to a total of 8 disks.

Choosing 6 or 7 disks by incorrectly assuming all data can reside in a single large RAID 5 array or miscalculating the parity overhead for multiple smaller arrays.

Community Discussion (5 comments)

ibn_e_nazir 👍 1

For RAID 1 we need 2 disks as it works on mirrored volumes To Calculate the Number of Disk for RAID 5, we need to calculate like this Useable Capacity = (Number of disks -1) x Disk Size Or Useabel Capaciity= (N-1) x Disk Size Here Useable Capacity = 20GB Disk Size = 10GB N=? Place the values in the formula 20GB = (N-1) x 10 20GB = 10N-10 10N =20+10 10N = 30 N = 3 Same calculations will be performed for the 2nd RAID 5 array Final calculation will be = 2 + 3 + 3=8

cj207800 👍 1 Selected: C

RAID 1 for Log Files: 2 disks RAID 5 for Data Files: 3 disks RAID 5 for Operating System: 3 disks

ccoli 👍 2

C is not correct, it does not specify that the 2x 20gb of Raid5 need to be on separate arrays so it is 2 disk for the raid1 = 10gb and 5 disk of raid5 = 40gb.

surfuganda 👍 1 Selected: C

C. 8 Disks (per AzadOB's explanation)

AzadOB 👍 4 Selected: C

To calculate the minimum number of disks needed for the given RAID configurations, let's break down each requirement: RAID 1 for log files: This requires mirroring, so each disk contributes half of its capacity. Since we need a total of 10GB usable space, we'll need 20GB raw capacity (10GB mirrored). Thus, we need 20GB / 10GB = 2 disks. RAID 5 for data files (20GB): RAID 5 requires at least three disks, and one disk worth of capacity is used for parity. So, for 20GB of usable space, we'll need 3 disks. RAID 5 for the operating system (20GB): Same as the data files, we'll need 3 disks for 20GB of usable space. Adding these up: RAID 1 (log files): 2 disks RAID 5 (data files): 3 disks RAID 5 (operating system): 3 disks Total disks needed: 2 + 3 + 3 = 8 disks. So, the correct answer is: C. 8

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Expert Analysis

Why the Answer Is Correct

To meet the requirements, we must calculate the disks needed for each independent array. For the 10GB RAID 1 log files, since RAID 1 mirrors data, you need 2 disks of 10GB each to get 10GB of usable space. For the first 20GB RAID 5 data array, the formula is Usable = (N-1) DiskSize; thus, 20 = (N-1)10, meaning N=3 disks. For the second 20GB RAID 5 OS array, the same logic applies, requiring another 3 disks. Total: 2 + 3 + 3 = 8 disks.

Why the Other Options Are Wrong

Option A (6) and B (7) are incorrect because they likely result from attempting to consolidate the two 20GB RAID 5 requirements into a single array or underestimating the redundancy overhead. Option D (9) is unnecessary as it exceeds the minimum requirement derived from the standard formulas.

Community Comment Notes

Comment [1] correctly breaks down the math for each segment. Comment [2] raises a valid point about array separation but ultimately agrees with the calculation logic implied by answer C. Comments [3] and [4] provide clear step-by-step derivations confirming the 2+3+3 split.

Official Reference

Array

Exam Strategy

Always treat different RAID level requirements as separate physical arrays unless the question explicitly mentions software-defined storage or pooling. Memorize the basic capacity formulas: RAID 1 is 2x disks for 1x capacity, and RAID 5 is N disks for (N-1)x capacity.

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