Why does a vSAN cluster need slack space?

I usually get a lot of questions during trainings or in the process of vSAN designs. People ask me why there is a requirement for 30% of slack space in a vSAN cluster. If you look at it without going deeper, it looks like a waste of (expensive) resources. Especially with all-flash clusters it’s a strong cost factor. Often this slack space is mistaken as growth reserve. But that’s wrong. By no means it’s a reserve for future growth. On the contrary – it is a short term allocation space, needed by the vSAN cluster for rearrangements during storage policy changes.

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