I tried googling that. Oy vey! There are some very opinionated people
out there on this. One editor was adamant that raid 6 will stop
functioning in 2019 based on the growth in size in disk drives. Of
course, he also predicted that raid 5 would stop working in 2009.
When one adds a drive to a raid 5 set, or rebuilds a replaced drive in
that same set, you are going to notice the performance hit. Much like
some people say you can really notice if you change your cache batteries
live during the day. (Like the last time our Mimix replication really
started to back up.)
There are a lot of people who say that they have experienced a second
drive failure while rebuilding their raid 5 stripe after a drive
replacement. And they can show you mathematically why this is likely.
Especially as drive size increases.
Drives aren't as expensive as they used to be. And we are used to paying
more for commercial grade drives than consumer grade drives (and I'm not
just talking about IBM Power based drives). But what you also have to
consider are the cages, power consumption, cooling and other expenses.
Basically why a lot of people don't mirror large arrays.