Exam 70-740 topic 1 question 205 discussion

Everyone seems to be confusing enclosures with physical disks, the question states... "Storage Spaces must remain available if two enclosures fail. The solution must minimize the number of enclosures required" It's obviously not going to be one disk per enclosure. So number of disks is irrelevant to this question as it is not asked, what it asks is.. Which resiliency type? - Three Way Mirror (we all agree on that). Number of Enclosures? - Not disks, the reason it is asking about enclosures is because the question is about enclosure awareness. You need at least 3 enclosures for enclosure fault tolerance. Mirroring is required for VM storage. 2-way mirror gives you 50% of raw capacity as usable storage. 3-way mirroring offers 33% of raw capacity as usable storage. 3-way mirroring with enclosure awareness stores each interleave on each of 3 enclosures (2-way does it on 2 enclosures, but you still need 3 enclosures for enclosure fault tolerance).

Got that q on exam

Read the question friends. The scenario MUST remain available if TWO ENCLOSURES fail. Not disks, not columns, but Enclosures. Under those requirements, you have to have 5 enclosures. Yes, you can have three-way mirror with three enclosures, that is the minimum. But they are not asking for the minimum. The "fastest possible write performance" is mostly a herring pointing you away from parity. See the "What types of storage arrays can I use with Storage Spaces?" section in reference: https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx

Yes, 3 and 3 - 3-way mirror is clear. 3 enclosures not so clear, but we are not talking disks, so each enclosure already has fault tolerance within itself. 33.3% storage efficiency across all 3 enclosures. Each enclosure has a full copy of the mirrored data. So 3 enclosures is minimum.

https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx#Enclosure_Awareness_Support_Tolerating_an_Entire_Enclosure_Failing https://docs.microsoft.com/en-us/windows-server/storage/storage-spaces/storage-spaces-fault-tolerance#examples 1st link states the recommended - 5 enclosures 2nd link states the minimum that 3-way-mirror needs to work and be safe - 3 enclosures. Now both are MS links and contradict each other. From my point of view is that 3 enclosures are doable and safe but not recommended, as 5 enclosures will be optimal and do not require immediate actions from admins. Question asks for minimum so I will go with 3 enclosures.

I found this below: Enclosure Awareness Support - Tolerating an Entire Enclosure Failing To support deployments that require an added level of fault tolerance, Storage Spaces supports associating each copy of data with a particular JBOD enclosure. This capability is known as enclosure awareness. With enclosure awareness, if one enclosure fails or goes offline, the data remains available in one or more alternate enclosures. To use enclosure awareness with Storage Spaces, your environment must meet the following requirements: JBOD storage enclosures must support SCSI Enclosure Services (SES). Storage spaces must use the mirror resiliency type if you're using Windows Server 2012: To tolerate one failed enclosure with two-way mirrors, you need three compatible storage enclosures. To tolerate two failed enclosures with three-way mirrors, you need five compatible storage enclosures." https://social.technet.microsoft.com/Forums/windows/en-US/db5c36d4-a83c-419c-b32a-e50880d84124/storage-spaces-enclosure-awareness?forum=winserverfiles

I am confused about the minimum number of enclosures. In Alex121 link it says clearly "To tolerate two failed enclosures with three-way mirrors, you need five compatible storage enclosures" but on the table below, there are 2 columns, one for 3 enclosures and one for 4. Also I have found these: https://aidanfinn.com/?p=17395 It says: "Decide your enclosure awareness (fault tolerance) and data fault tolerance (mirroring/partity). You need at least 3 enclosures for enclosure fault tolerance. Mirroring is required for VM storage. 2-way mirror gives you 50% of raw capacity as usable storage. 3-way mirroring offers 33% of raw capacity as usable storage. 3-way mirroring with enclosure awareness stores each interleave on each of 3 enclosures"

Which resiliency type? - Three-Way Mirror Number of Enclosures? - 5 "To tolerate two failed enclosures with three-way mirrors, you need five compatible storage enclosures". https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx#Enclosure_Awareness_Support_Tolerating_an_Entire_Enclosure_Failing cheers

So this is 3 Way mirror with 5 Enclosures?

answer should be: Three-way mirror and requires minimum of 5 disks...it writes 3 copies of your data to protect against 2 simultaneous drive failures thus requires at least 5 disks

The reason is that a 3-way mirror uses a quorum. In order to keep running, the mirror space must keep over 50% of the disks functioning. So a 3-way mirror must have at least 5 physical disks to be able to survive the loss of up to 2 physical disks. A 2-way mirror does not require this type of majority vote.

For 3 way mirror you need 5 disks

Why is the number of enclosures 3? I do not understand that part.