Disk space allocation in RAID 5


I have a server with 3 x 72.8 disks in it. The array is configured for RAID 5, so technically I get 2 x 72.8 = 145.6 GB of space.

However, the Array Configuration Utility states I have 135 GB, and also the sum of all the partitions is equal to 135.

Where does this missing space go?

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nplibConnect With a Mentor Commented:
Hardware based raids use more disk space for it's allocation table, unlike a normal Partition table that only uses about 512 KB of the disk, a RAID 5 table uses a percentage of the disk for it's allocation table.

A Software based raid uses less disk space, but is less stable and if the OS crashes, you loose everything.
First just because a drive says it's 72 GB doesn't mean it really is. You could buy a HD that on the box says 72 GB but when windows is done formating it is 69 GB for example.

Second You lose a bit of space about 5% ish when creating a raid 5. Not only does it use a1/3 of it's drive for parity, but it uses that space as like storage for a table that tells the drive where all the bits are kept. So if you lose a drive, you won't loose the data. This way even with a drive removed the computer can still function as normal.
Also, whether it's done with Hardware or software makes a difference as well.
Is your raid software based or hardware?
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kam_ukAuthor Commented:
The RAID is hardware based...
kam_ukAuthor Commented:
Excellent, thanks very much!
In addition there is the rounding issus betwen 1 MB as 1000x 1000, vs
1024 x 1024. It adds up.

I hope this helps !
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