Expand LUN: concatenation or striping?

If you are expanding ‘within’ the same RAID Group, always use concatenation. If you stripe in the same RG it will cause head thrashing.

If you are expanding into a new equal size RAID Group, then striping is likely the best option, but it would depend a bit on all the particulars.

Richard J Anderson

Hi Richard,

my Raid Group has 1207 GB total and 402 GB free space, and i have only one LUN into this Raid Group whose size is 805 GB.

I want to expand this LUN, but when i choose Concatenation navisphere ask me for the unused LUN; i don't have any unsed LUN and i can't continue with this option.

Therefor i have only the striping option. My question is what happen with the data if choose striping, is it secure? This LUN stores Exchange Databases.

Thanks in advance

Erasmo

Before you expand, you need to first create a LUN in the free space. That new LUN will be concatenated onto the existing LUN during the expand operation.

Richard J Anderson

Sr SE - Western Division, EMC

richardj.anderson@emc.com

206.229.9263

EMCTA, SPEED

To answer you other question..  The expand operation is not-disruptive..

For concatenation, the new LUN is added to the end of the existing LUN and the free space is instantly available.

For striping, the existing data is restriped across the existing LUN and new LUN.  The new free space will not be available until the re-striping operating completes which could be several hours or longer.

Still, ONLY use concatenation when expanding inside the same RAID Group.  If two LUNs in the same RG are striped together, the disk head will have to seek between the two locations when accessing data that is stored together in the LUN.  Response times will be very poor.

Sequence of tasks would be something like the following..

1.) Expand RAID Group (you already did this)

2.) Defrag RAID Group (if necessary, check the General and Partitions tab in the RAID Group properties to ensure that all of the free space in the RAID group is contiguous.)  After RG expansion, sometimes you will have multiple gaps of free space, defrag moves all data to the start of the disk and makes all free space at the end.

3.) Create a new empty LUN in the free space that is the same size as the amount of space you want to add to the existing LUN

4.) Expand the existing LUN using concatenate, and choose the new empty LUN to use.

5.) Rescan the disks in Disk Management, then extend the NTFS partition.

Exactly..

At the command line you start diskpart, then you will need to select the disk and partition you are extending, then the extend command..

Something like…

C:\> Diskpart

Microsoft DiskPart version 6.1.7601

Copyright (C) 1999-2008 Microsoft Corporation.

On computer: list disk

Disk ### Status Size Free Dyn Gpt

it's not automatic, you will need to use diskpart in win2k3 ..in win2k8 you can use Disk Administrator

Richard,

thanks for your help, but what do you mean with rescan and then extend the NTFS partition? i thought the expansion would be automatic in Windows.

Thanks again

Erasmo

Richard/Dynnamox,

thanks a lot for your help it´s very clear.

Finally we have decided move the data into the LUN and then destroy it, then we'll create a new LUN with the total free space.

thanks again

Best regards.

Erasmo

why did you end up going that route ? What Richard described is a very common procedure, nothing exotic or dangerous.

Thanks for the clarification Glen

Please remember to markyour question answered when you get the correct answer.

glen

Just an added note. When you create a Raid Group, then you create a LUN in that Raid Group and assign the size as "Maximum", if you later expand the Raid Group by adding more disks, then the LUN will also expand to the new size. This ONLY works with a single LUN in the raid group set to "Maximum" size when you create the LUN.

Whenever you expand a LUN on the array that is attached to a host, the host operating system must do something to see the new size.

glen