PERC 6/i Specs

I think you don't seem to fully understand raid.

RAID 1 - 2 x 250 GB + hot spare (so 3 x 250 GB total, for OS)
RAID 6 - 4 x 750 GB + hot spare (so 5 x 750 GB total, for data storage)

A RAID 1 with 2 x 250GB = '250GB' of usable space

A RAID 6 with 4 x 750GB = '1500GB' of usable space ((number of drives minus 2) times the size of the smallest drive)

The specs say it supports up to 32 virtual disks. You seem to want to make 2 virtual disks, so you can easily do this.

Assuming that you're buying (or already own) a machine with 8 drive bays, such as a PowerEdge 2900 III, that setup will work.

 

 

However, RAID 6 has significant overheads - and RAID 6 on four disks with a dedicated hot spare seems like overkill. The chances of losing two drives from a four drive set are pretty low, especially as you have to lose those two drives in quick succession to destroy the RAID 6 virtual disk. The first failure will trigger a rebuild onto the dedicated hot spare, which will, if successful, return you to two drive redundancy.

 

RAID is never a substitute for proper backups.

 

 

If you go for your proposed setup, you could put one 750GB drive in as a global hot spare, and save a little money. It doesn't matter that the 750GB drive is larger than needed to be a spare for the 250GB drives int the RAID 1.

 

 

However, you may well want greater performance out of the OS / application virtual disk than 7200rpm SATA drives permit. If you have the budget, you could make the drives in your RAID 1 300GB 15krpm or 10krpm SAS drives. Mixed SAS and SATA systems do work - though Dell Sales tend not to permit you to buy a server in that configuration to keep the number of configurations down. The only restriction the PERC enforces is that you can't mix SAS and SATA drives within the same virtual disk.

 

 

The 2950 III that's currently my development machine (it will eventually go into production) has 2 x 300GB 15krpm SAS drives in RAID 1 for the OS and applications, and 4 x 1TB 7200rpm SAS drives in RAID 5 for data. In production, it will be included in an LTO-4 tape backup system, so I'm happy with single drive redundancy.

 

For a while, I had 4 x 80GB SATA drives in place of the 4 x 1TB SAS drives - the server was perfectly happy with this 'mixed' SAS and SATA setup.

It is indeed possible that the original poster hasn't understood RAID very well. The proposed scheme uses a lot of drives to give relatively little space considering the sizes of drives used. The performance of the OS / applications on a RAID 1 made up of 7200rpm drives is probably sub-optimal, too.

 

It would help if we had some idea what are the intended uses for this server. If the intention is large storage with a small number of drive, then maybe something more like my setup is appropriate - though there's nothing preventing the use of SATA 1TB or 750GB drives if that is desired, even though Dell Sales may be reluctant to sell a mixed SAS and SATA server.

 

 

Hot spares can be an important way to add extra protection to important virtual disks - but it's not that usual to see a dedicated hot spare used for a RAID 1.