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Upgrading a 3-disk RAID5 with an extra HDU - is data integrity retained?

Posted on 2013-05-27
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Last Modified: 2013-05-28
Hi,

I have a client with a HP Prolaint with a RAID5 configuration with 3 146GB disks. They have some problems with data usage and they want to expand the storage. Now someone has advised them to add one 146GB harddisk, and they claim that the data integrity will not be changed.
The client assumes that the 4th harddisk will be aded to the RAID5 config. If this can be done, he will not have a nett storage of 146GB but less, am I right? And is this a save sollution?

Thnx for your imput.

Grtz,

Erwin.
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Question by:Erwin in de Braekt
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by:rjanowsky
ID: 39200449
Have look at this thread concerning RAID expansion at the HP Communities. You can also check this HP Reference Guide for Array Configuration. Array expansion is possible, if some preconditions are met. And you are right, that not the complete capacity will be usable, because the RAID5 needs some space for redundancy.
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andyalder earned 1500 total points
ID: 39200724
Parity is maintained during expansion, it re-stripes from 3 disk RAID 5 to 4 disk RAID bit-by-bit* and keeps track of the progress using the battery backed write cache. Normal write cache is disabled during the expansion since the cache is used in the re-leveling process, you can even have a power outage half way through and because of the cache it will pick up again where it stopped. You should still backup first just for safety and continue with your normal backup regime during the expansion, it does take a long time but the server is still usable during the process. You can change the expansion priority using the ACU to make it expand faster at the cost of reduced performance.

He will have the full 146GB extra space (taking 146GB as 146000000000 bytes) since one disk worth of space is used with RAID 5 whether 3 or 4 disks in the array. The extra space won't be added to any logical disks you have but you can expand the current logical disk or create a new one after the expansion is complete.

*It may be that it does a single full-width stripe at a time or it may do several at once, I'm not sure about that.
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