Minimum Disks for Mixed RAID 1 and RAID 5 Configurations

SK0-005 By islever Editorial Last updated Aug 20, 2026

Short answer

C 8

Question

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 Correct Answer
9

Key Takeaways

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.

Common mistake: 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.

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

This link opens the official vendor documentation in a new tab.

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.

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.

Discussion 5 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 5

AzadOB 👍 4 Answered 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
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.
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 Answered C

RAID 1 for Log Files: 2 disks RAID 5 for Data Files: 3 disks RAID 5 for Operating System: 3 disks
surfuganda 👍 1 Answered C

C. 8 Disks (per AzadOB's explanation)

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