EVA4000: Where did all my disk space go?

[CA806597]

Hi,
I'm no SAN admin.
I got 8x146GB HD and gave them to the SAN ppl to configure RAID5 for me and I ended up with 650GB usable space. They used "single drive failure" or something. They couldn't explain to me where all the other space went and I kept looking for a document that could explain what EVA does with the disks.
Anyone has a pointer to a good document or can give me a quick explanation?
thanks

[BR894765]

Single protection level reserve space of two disks. EVA RAID5 have 4+1 scheme (4data+1parity). Some space gone in marketing-to-realworld conversion (HDD manufacturers counts 1GB=1000MB=1000000KB, when OS assumes that 1GB=1024MB, 1MB=1024KB).
So (8*146-2*146)/5*4*conversion=650,85GB

[BR894765]

Two disks reserved because EVA must relocate both disks in pair in event of RAID1 failure.

[CA806597]

Thanks Basil,
just one more question.
I'm using RAID5, do I still have to use this "Single preoction" level feature?
If I don't use that what's the worst that could happen? Wouldn't I still be protected by the RAID5 scheme offering protection for one failed disk (for each four)?