A misunderstanding of this overhead, has caused some people to recommend using “(2^n)+p” disks, where p is the number of parity “disks” (i.e. 2 for RAIDZ-2), and n is an integer. These people would claim that for example, a 9-wide (2^3+1) RAIDZ1 is better than 8-wide or 10-wide. This is not generally true. The primary flaw with this recommendation is that it assumes that you are using small blocks whose size is a power of 2. While some workloads (e.g. databases) do use 4KB or 8KB logical block sizes (i.e. recordsize=4K or 8K), these workloads benefit greatly from compression. At Delphix, we store Oracle, MS SQL Server, and PostgreSQL databases with LZ4 compression and typically see a 2-3x compression ratio. This compression is more beneficial than any RAID-Z sizing. Due to compression, the physical (allocated) block sizes are not powers of two, they are odd sizes like 3.5KB or 6KB. This means that we can not rely on any exact fit of (compressed) block size to the RAID-Z group width
