EVA 5000: Vraid5 overhead

[Pearl Park]

I have a question about EVA 5000.
What is the overhead for VRAID5 redundancy ?
EVA InfoPak dated May 2003 says that Vraid5 consumes 1.25 blocks of physical capacity for every block of usable capacity.
However, when I see Command View EVA 3.1's(VCS version 3.010) report, it seems that Vraid5's overhead is 20 % of usable capacity, not 25 %.
Can anyone tell me what is correct ?
Thanks.

[Uwe Zessin · Accepted solution]

Both are correct ;-)

4 data segments are combined with one parity segment: (4+1)/4 = 1.25

Assuming you have 100 'raw segments', then 80 segments will be used for data and 20 for parity: 100% / (4+1) = 20%

[Steven Clementi]

Pearl:

I can confirm that the VRAID overhead is @20%. On all the EVA's I have installed over the past 3 years, it's been @20%.


Steven

[Pearl Park]

Thanks for replies.

[Pearl Park]

Responses offer excellent explanation.
Now I have the correct formula to compute available space for Vraid5 disk.

[Steve Cavendish_1]

You may have the correct formula for the RAID level but do not forget about the Hex to decimal conversion. Your 146GB harddrives actually only have 142GB or so in Hex. Hard drive maufacturers base their capacities on 1000MB=1GB not 1024MB=1GB in large RAID array's it make a big difference. Also read up on the EVA if you already haven't. Spare capacity works different and you should not exceed 95% of available capacity of leveling performance.