RAID 5 = Disk Usage Question

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TJ

Hi,

We just got a machine that has about 4 SATA drives on it, and it does
a software RAID 5.

The problem is, the machine has 4 drives, making it up 1 terabtye
machine, but due to the software raid, when i create a partition, i
can see maximum of about 650 GB's. Is this normal in such situations?

How much space would a typical software raid take (apart from the OS)

Thanks,

TJ
 
Hi,

We just got a machine that has about 4 SATA drives on it, and it does
a software RAID 5.

The problem is, the machine has 4 drives, making it up 1 terabtye
machine, but due to the software raid, when i create a partition, i
can see maximum of about 650 GB's. Is this normal in such situations?

How much space would a typical software raid take (apart from the OS)

Thanks,

TJ
Click to expand...

With RAID 5, either accomplished in hardware or software, the theoretical
maximum storage efficiency is (number of drives -1) / (number of drives)
which in your case would be 3/4 or 75% at least assuming that the system has
4 drives and not just "about" 4 drives. Then you have to deal with OS
formatting overhead. And then there is the matter of number juggling in the
manufacturer's drive size specification, i.e. does gB=1,073,741,824 bytes or
does gB=1,000,000,000?

If the system were mine I probably wouldn't be dissatisfied with 650gB no
matter how it got there but if you consider that RAID 5 can be accomplished
with just 3 drives you might be getting exceptional storage from the system.
Go and count the drives and check what the capacity of each is and and
determine what the manufacturer thinks a gB is and then you'll know how well
you're doing.
 
In a RAID 5 array, if you have four drives, a little over 1/4 of the
harddrive space is used to store the checksums that are used to reconstruct
the data if one of the harddrives fails.
 
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