Pegasus2 not migrating from RAID 5 to RAID 10

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Ian MacDonald posted this 5 weeks ago

I have four 4TB drives - currently in a RAID5 configuration (8TB capacity)

I want to migrate to a RAID10.

The manual says it should be possible, but when I go to Background Activities|Migration I only see RAID 0 & RAID 5 as options. 

Can anyone help me understand why I cant see RAID10 as an option?

Thank you!

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PROMISE Technology Inc. posted this 5 weeks ago

Hi Ian,

I don't know how many drives your Pegasus has, but the following is true for 4,6, or 8 drives.

A 4-drive RAID5 has 3 drives worth of data and 1 drive of parity. For 4TB drives this amounts to a 12TB LUN.

A 4-drive RAID10 has 2 drives worth of data and 2 drives of parity. For 4TB drives this amounts to an 8TB LUN.

You cannot convert a 12TB LUN to an 8TB LUN, the data blocks cannot all map to a smaller LUN.

You can go from RAID5 to RAID10, but they have to be the same size. If you had a 3-drive RAID5, you could convert that to a 4-drive RAID10 as they have the same number of data blocks. Every block in the RAID5 will map to a block in the RAID10. But it is not possible to convert any 4-drive RAID5 to a 4-drive RAID10, the same is true for 6 and 8 drive RAID5 LUNs.

Note that the math is different for 6 and 8 drive arrays, but the effect is the same.

If you want a RAID10 your option is to offload the data, delete the R5, create a new R10 and copy the data back.

Ian MacDonald posted this 5 weeks ago

Hi - thanks for the fast reply.

It is a R4 Pegasus2 with a 4-drive RAID 5.

I had assumed as the Logical drive is only set to 8TB that I could do a migration from an 8TB RAID 5 to a 8TB RAID 10...

If not - then I will do asyou suggest and create a new R10...

PROMISE Technology Inc. posted this 5 weeks ago

Hi Ian,

According to the Product manual page 103, a R5 > R10 migration is possible.

But I think the problem here is units. A 4TB drive is only 4TB in the metric system, a computer sees 3.63TB. So the actual capacity of a 4-drive array is 4 * 3.63 = 14.52 TB. The capacity of a 4-drive RAID 10 would be 2 * 3.63 = 7.26 TB, so your logical drive is larger than will fit in a 4-drive RAID10.

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