Can't see extra space from growing LUN?

[shorton]

     VSphere 4.0 resized my 512GB SAN LUN to 1.5TB.  VC recognizes LUN as 1.5TB but existing Virtual Servers cannot see resized LUN still sees it as 512GB.  How do you get existing VM's to recognize resized LUN's?

[Troy_Clavell]

what is is you are trying to do?  You resized your LUN, you are also trying to resize a HDD presented to your guest?

[shorton]

Yes I want to resize one of the disks presented to this VM from 50GB to 1.5TB but not lose the data already on it.  

[mittim12]

Your maximum disk size is going to be dependent on the block size of you VMFS datastore.  Do you remember what you set that to? 

[Troy_Clavell]

ok... WIndows guest or Linux?  Once we understand the guest OS, then we'll see what the best method to increase the drive in question is going to be.

[Troy_Clavell]

...and yes, if that LUN is set with a 2MB block, the maximum you'll be able to increase is 512GB

[shorton]

2008 R2 is the VM OS, On this ESX host If I add a new VM to this LUN I can create a 1TB or larger Disk, I cannot add a disk larger than 512GB to this existing VM. thx

[Troy_Clavell]

it's probably a block size issue

http://kb.vmware.com/kb/1003565

[mittim12]

Do they live on the same datastore?   You can view this KB for block size information, http://kb.vmware.com/kb/1003565.   If it is indeed the block size you would need to storage motion to another datastore with a large block size to increase the size.

[shorton]

Ok if I change the block size I should automatically see the change in my VM then?

[Troy_Clavell]

Ok if I change the block size I should automatically see the change in my VM then?

yes, but changing the block size will require a format of the LUN and new presentation.

[shorton]

No problem I'll storage vmotion the HD to a different LUN then Represent the 1.5TB LUN and reformat with 8MB Block. thx!

[shorton]

That worked! Thanks so much for your help! Scott

[mittim12]

That's great news.  Since the inception of the LUN expansion ability there has been a lot of talk about block size.  From what I gather it doesn't hurt to set all of your block sizes to 8MB from the start now.   Troy, have you heard different or found a reason to not do this? 

[shorton]

What about using LUN's larger than 512GB?  I have always been told this is best practice? thx.

[Troy_Clavell]

there has been debate about whether or not a higher block size affects performance, although there really hasn't been a definitive answer.  It was talked about awhile back at Duncan's site

http://www.yellow-bricks.com/2009/05/14/block-sizes-and-growing-your-vmfs/

http://www.yellow-bricks.com/2009/03/24/an-8mb-vmfs-blocksize-doesnt-increase-performance/

[mittim12]

LUN sizes are always a little different because it depends on what you put on the LUN.  With the improvements in offloading task to the array and some of the reservation locking you can get away with putting more VM's on a LUN.  I think the bottom line is to simply  make sure you have the IOPS to handle the VMs.  

[AureusStone]

If you are not using VAAI, then you should still consider locking to be very important.  Having said that I can't see any issues with having a big lun like this with one big VM.

[depping]

Well Locking has been improved as well vastly over the year with what we  call "optimistic locking" and hasn't been that big of an issue. The real question is wether you can afford to have a LUN which is 2 x the size of your current LUN from an availability perspective. As the impact of that LUN getting corrupted is a lot bigger. Also the time it will take to restore will increase.

Duncan (VCDX)

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