Calculating ASM diskgroup Usable Size

Sometimes sizing information of an ASM diskgroup returned by “lsdg” command from “asmcmd” prompt (similar information is also available under v$asm_diskgroup view) leaves DBAs perplexed. Sizing information and usable space for diskgroups with EXTERNAL REDUNDANCY is quite easy to understand, but when it comes to NORMAL or HIGH redundancy diskgroups, it is not very straightforward. Here I would explain ASM diskgroup space calculation in detail to make it easier to understand. Before I explain how to calculate usable space in an ASM diskgroup, some useful information to note is as follows