i try to install ESXi 5.0 but i can't downgrade the block size from 1MB to 4 Kb. Why?
it's default in ESXi5 and i do not see a reason of downgrading. for lower block it is advisable to format it from version 4 then upgrade to version 5 but then this is waste of time as the different block levels does not have performance difference
it's possible tu upgrade from 1MB to 4 MB??
vsphere5 has a unified support of 1MB block only natively, unless the datastore is inherited from vsphere4
There's no need to up- or downgrade the VMFS block size in VMFS-5. As already mentioned the unified block size is 1 MB and cannot be modified. VMFS-5 no supports LUNs with up to ~64TB with this block size. In addition to supporting larger LUNs, the sub-block size has been reduced from 64kB to 8kB.
see e.g. http://www.yellow-bricks.com/2011/07/13/vsphere-5-0-what-has-changed-for-vmfs/ for more details