RAID 5: 3TB raw storage, how much usable can I expect?

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Level 2

357 points

RAID 5: 3TB raw storage, how much usable can I expect?

Been trying to figure this out through googling, have gotten conflicting answers:

I have an external RAID tower with 6x500GB drives, so 3TB raw. If I go RAID 5, I should have 2.5TB usable space, right? And then when I initialize in Disk Utility will I lose just a little bit of space or a whole lot more?

I'm having issues after striping the RAID; the software reads 2.5TB usable but Disk Utility only shows 2TB usable when I format it. Someone has told me that's all I should expect, but that doesn't seem right -- losing a third of my storage??

MacPro 2.66, G4 Dual 1g, 15" Powerbook 2.66, 8GB iPhone, iPod video, Mac OS X (10.5.6), GSPEED 3TB

Posted on Feb 24, 2009 1:11 PM

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Answer 1

The hatter

Level 9

61,031 points

Feb 24, 2009 1:26 PM in response to John Knowles

http://en.wikipedia.org/wiki/StandardRAID_levels#RAID5

http://support.apple.com/kb/TA24883

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Answer 2

The hatter

Level 9

61,031 points

Feb 24, 2009 1:45 PM in response to John Knowles

Also, take into account:

500GB = 465GB * .8 = 372GB

Utilize no more than 80% of your volume.

2232GB is the maximum real storage space.

I would agree with DU, only 1.8TB of usable space.

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Answer 3

John Knowles Author

Level 2

357 points

Feb 24, 2009 1:46 PM in response to The hatter

Thanks for that. So according to formula in the wikipedia article, (N-1)*Smin, I should be seeing 2.5TB

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Answer 4

John Knowles Author

Level 2

357 points

Feb 24, 2009 1:57 PM in response to The hatter

Wait, how do you get the 500GB = 465GB? You lose 7% before you calculate the 80%?

The main reason I'm suspicious about this is that the RAID GUI has a lot of selections to enable support for arrays over 2TB; it's odd that DU reports exactly 2TB availble, as if it was cutting it off (rather than show 2.3, 1.8 or some other number; it's exactly 2TB ). Methinks the +2TB is not taking; I have a call into tech support...

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Answer 5

Feb 24, 2009 2:35 PM in response to John Knowles

Hi John Knowles;

A 500 GB drive in salesmen speak give you 465 GB in computer speak.

Allan

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Answer 6

direwolf8

Level 4

1,286 points

Feb 24, 2009 2:46 PM in response to John Knowles

You're looking at raw storage on the drive. The filesystem takes up space, so you'll use a chunk of it before you store a single file.

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Answer 7

The hatter

Level 9

61,031 points

Feb 24, 2009 3:29 PM in response to John Knowles

If you are doing RAID5, that also means you are using 3rd party controller (Apple Pro RAID is internal and doesn't support 6 drives; my HH RR2314 is 4-channel but has Port Multiplier I think, never used it that way) and supports RAID5.

Maybe the card firmware is limited to 2TB volumes?

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Answer 8

John Knowles Author

Level 2

357 points

Feb 24, 2009 5:33 PM in response to The hatter

Yeah I have an ATTO SCSI card, but it doesn't seem to be limited to 2TB; the company offers options of 6GB and higher with the same card.

I've been checking the math and I think you guys are right: 2.5TB of raw space in RAID 5 minus 20% for formatting is 2TB on the nose.

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Answer 9

mbean

Level 4

1,250 points

Feb 24, 2009 6:42 PM in response to John Knowles

I've been checking the math and I think you guys are right: 2.5TB of raw space in RAID 5 minus 20% for formatting is 2TB on the nose.

The RAID 5 should format to approximately 2.2TB with six 500GB hard disks.

If the RAID volume only shows 2TB my guess is that either the driver has a 2TB limit or when you configured the RAID with the web manager over 2TB support was not selected.

The Areca cards have this same limitation. You have to set the web manager to support volumes over 2TB when configuring the RAID set.

Have fun!

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Answer 10

John Knowles Author

Level 2

357 points

Mar 2, 2009 10:57 PM in response to mbean

Solved the mystery.

I formatted as RAID 0 to check the results and it also cut off at 2TB, so I knew there was a problem.
The GUI had different (and obscure) settings for supporting over 2TB; tech support (G-technology) had me try a different one from what the manual stated and the RAID 0 volume showed up as 2.7TB in Disk Utility.

Whew. Thanks all for the help.

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