MSA Page size is 4MB and in VMware ESXi 6.5 there is a limitation of if any array unmap granularity is greater than 1MB then automatic unmap not supported. For VMFS6, reclamation granularity equals the block size. When you specify the block size as 1 MB, the granularity is also 1 MB. Storage sectors of the size smaller than 1 MB are not reclaimed. Please find the MSA best practice guide to understand Block size/Page size and also the VMWare link to understand how unmap and space reclaim works,
https://www.hpe.com/h20195/v2/getpdf.aspx/4aa4-7060enw.pdf (page 51)
