Mike: Unfortunately, it's not quite as simple as just adding all the storage up mathmatically as Donald indicated. It would be, with just raid-5, but this is autoraid, which also steals away some space for its own nefarious purposes (actually beneficial ones, but, I digress).
I'm not saying that 8 9GB drives won't work, especially since the math comes out slightly over 50GB. Maybe all that the "autoraid" functionality requires is the extra, and you'll end up with 50GB, spot on. What I'm saying is, don't guarantee it, there are some intangibles with autoraid.
What your client WILL see, as they use the storage, is that they will have very decent performance, at first, before you fill all 50GB (or whatever proves to be available). When an autoraid unit has extra space, it stores all current data as RAID 0/1 (mirrored), for best performance. As you start to fill it closer and closer to capacity, more and more data is migrated to RAID-5, to make additional room. This migration requires some buffer space, hence the "hidden space tax" of autoraid.
Your client will probably begin to notice a performance dropoff as the unit gets nearly full, just how much depends on the "working set", the data that is touched in any 24-hour period (for instance). The 12H units migrate data from mirrored to raid-5, until they reach a minimum of 10% of the available storage as mirrored. So, you'll never have an "all raid-5 array", it can't be done. And, for many users, they only touch 10% of their stored data in any 24-hour period anyway, not including backups.
The beauty of autoraid is that, if your client sees a performance dropoff, as the unit gets fuller, they're likely to not like it (performance is adictive, and you can never go backwards, only upwards...) So, to solve their performance problem, you just slip in another drive of whatever type you go with. The 12H sees more free space, and begins to migrate most recently used data back to mirrored/raid-0/1 storage. Performance goes back up, and all is well. This is without allocating anything. The extra storage goes into extra "mirrored" space, unless you allocate it for use.
So, when you get this thing set up, you can use the 'arraydsp' command (see man page for details) to see how much usable storage you actually get from 8 9GB drives. It will be something around 46-50GB, is my guess, and then you'll decide if it's enough, or if you need one more drive.
One other option is to turn off "hot spare" mode, which frees up the space of one drive for data. I recommend against this for two reasons. First, the space is used for extra "mirrored" storage when all drives are functional, for improved performance. Second, this is a nice "high availability" feature, and the price of one drive is very cheap insurance against data loss. With the spectre of data loss so costly in terms of downtime and potential lost data (depending on backups, etc), this "insurance" cost is very easily justified, in my book.
Best Regards,
--bmr
We must indeed all hang together, or, most assuredly, we shall all hang separately. (Benjamin Franklin)
