What happens enlarging a VMFS lun beyond 2Tb?

[enicon]

Hello,

I need some opinions about what happens if one:

• creates a LUN smaller that 2Tb (say, 1Tb)

• builds a VMFS on it and put there many VM

• with VMs running, goes to the storage array and resizes the lun, making it BIGGER that 2Tb

I know it is not a smart thing to do and that there is no reason to do it, but I want to understand what happens then from a strictly tech point of view.

For what I know about scsi, I guess that the vmfs will continue to work because there is no way to enlarge it to fill the new size, so it will stay within the 2Tb limit and conntinue to work.

But is it possible that the growth triggers some modification in the lun so that esx will no more be able to access even the first 2Tb, rendering the data unreadable and causing errors such as metadata corruptions in the vmfs?

I need a scsi expert opinion on that!

Thank you very much for any info and opinions.

Massi

[prakashraj]

Hi,

I am not the storage expert

But I heard from my trainer is that if we assign more than 2TB for example if 3TB then the server will assume only 2TB from that (Usable size will be 2TB)

Prakash

[enicon]

Thank you Prakash, that complies with a test I did on an XIV storage system. Do you remember who your trainer was, maybe I can ask directly to him? I'd love to get an opinion from some storage guys from vmware.

[depping]

2TB-512Bytes is the limit of VMFS for a single non extended volume. If you need or want more you can either create a second volume or add an extend to the original volume. If you resize your LUN nothing will happen as the VMFS volume has already been created with a fixed size. With vSphere you will have the option to resize the VMFS volume but also not outside the 2TB-512bytes region.

If you would create a new 3TB LUN and format it with VMFS you will end up with a 1TB volume instead of a 2TB or 3TB one.

Duncan

VMware Communities User Moderator | VCP | VCDX

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[depping · Accepted solution]

And no growing it will not corrupt your LUN. I personally don't see why you would want to by the way. Creating a secondary LUN and loading it with VMFS is a lot easier.

Duncan

VMware Communities User Moderator | VCP | VCDX

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[enicon]

For me too, it is a pointless thing to do. Personally I would have created another lun and put a new datastore on that. It was done by mistake by one of my customer, I'm just trying to understand what the implications of such an action would be on the preexistant data in the vmfs.

So you agree with me that the vmfs should not suffer from the enlargment.

Thank you very much

[enicon]

If you would create a new 3TB LUN and format it with VMFS you will end up with a 1TB volume instead of a 2TB or 3TB one.

Why 1 Tb? Shouln't I get 2? And I believe the lun would not show up in the "add storage" wizard. Of course one could try to do that by cli..

[Texiwill]

Hello,

If you have a VMFS on the LUN that is 1TB and grow the LUN to 3TBs you will still have a VMFS with only 1TB. The reason is that growing the LUN does NOT grow the VMFS.

If on the other hand you have a 3TB LUN and format it as VMFS,...

bq. Why 1 Tb? Shouln't I get 2? And I believe the lun would not show up in the "add storage" wizard. Of course one could try to do that by cli..

Because the size rolls over. VMFS can only be 2TB so if you have 3 TB is takes the left over from 3 - 2 and that is 1TB.

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[RParker]

I'm just trying to understand what the implications of such an action would be on the preexistant data in the vmfs.

Read up on LVM (logical volume manager in Linux). That explains it. You would have to create an extent to make the volume bigger, not sure how many extents a single volume can have (it adds free space to an existing volume and makes it bigger by adding disks).

I believe you can grow 8 extents of 2TB each, making a 16TB single volume, so in essence a SINGLE volume can't be that big but multiple volumes can act as 1.

[RParker]

Because the size rolls over. VMFS can only be 2TB so if you have 3 TB is takes the left over from 3 - 2 and that is 1TB.

That explains why a 2.5TB volume is 500GB, but why not simply drop the extra and use up what you can, and ignore the rest.

[enicon]

I know about extents, they are the obvious right way to grow a vmfs if one wants to. The question is what happens to an existing vmfs if I grow the lun (which IS a pointless thing to do )

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[AndreTheGiant]

You cannot grow a VMFS volume to more than 2 TB - 512B.

To have a VMFS volume bigget than 2 TB you have to use extent.

Andre

[depping]

Like I already said, if you grow the LUN NOTHING will happen to the VMFS Volume. You need to manually extent it before anything happens.

Duncan

VMware Communities User Moderator | VCP | VCDX

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