Usable Capacity of P4500 - Lower than expected

[Nick Salty]

I have just purchased a P4500 7.2TB unit to provide iSCSI storage for a dev environment.

I have just run it up, and accepted the default of RAID 6.

However, the usable capacity is showing as only 4.3TB.

The device has 12x600GB drives, which are showing as 558.98GB in the CMC.

Using RAID 6, the capacity should be reduced by 2 drives, which should give me a working capacity of 10 x 558.98 which is 5.6TB

What am I missing?

Nick

[Joris Vliegen]

Hey Nick,

The problem is they advertise space as 1 GB being 1.000.000.000 Bytes. While in reality 1 GB is 1.073.741.824 Bytes.

On this link your disk is advertised as having 600.127.266.816 bytes. When you divide 600.127.266.816 by 1.073.741.824 you get 558,91 GB.

So this little marketing thing adds up on big storage boxes.

Kind regards,

Joris

[Nick Salty · Accepted solution]

Hi Joris,

Thanks for your reply. I think that this kind of slack advertising is really not good enough when selling storage to business and enterprise.

My question goes further than this though. Even with disks of 558GB, a RAID 6 volume should still give me the capacity of 10x558GB= 5.58TB, and yet it does not.

However, since I posted this question, I think I have found the answer and that appears to be that TWO raid 6 sets are created, with 6 disks each. This means that I lose the capacity of 4 disks, not only 2.  The capacity of the 8 remaining disks would be about 4.46TB and hence usable capacity of about 4.3TB.

So the bottom line is that the 7.2TB device will actually only provide 4.3TB.

This is not suitable for my purposes, so I deleted the management group, wiped the node and started again using RAID 5. In the management group setup I selected to keep my current RAID configuration, and now have usable space of 5.44TB.  (The capacity of 2 disks are still lost as two RAID 5 sets are created, leaving me with 10x558GB)

So, buyer beware, and buyer calculate before purchase!

Nick

[ccavanna]

When we first went with p4500 storage we ran into the same issue. Now its is known on how much we loose and what we need to get when purchasing new storage. I do agree that their marketing is misleading.