Max LUN size and max RDM size?

[rickardnobel]

From the Configuration Maximums on 4.1 there seems to have been a change in the max LUN size, from the somewhat strange 2TB-512 byte to just 2TB. Can anyone confirm that this is correct in 4.1? From what I understand in 4.0 if you got your LUN created at exactly 2TB you lost around 500GB of space(?).

It would be interesting to know if this is "fixed" in 4.1 and if only using 4.1 hosts then 2TB LUNs could be created?

Also, the RDM maximum size seems to still be 2TB-512 byte in 4.1. What is the reason for this kind of limit? If the guest OS could accept larger disks than 2TB, should that not be enough?

[vmroyale]

Hello.

From the Configuration Maximums on 4.1 there seems to have been a change in the max LUN size, from the somewhat strange 2TB-512 byte to just 2TB. Can anyone confirm that this is correct in 4.1? From what I understand in 4.0 if you got your LUN created at exactly 2TB you lost around 500GB of space(?).

Interesting find. I wonder if that is a mistake? I don't recall seeing anything in the release notes about a change there.

It would be interesting to know if this is "fixed" in 4.1 and if only using 4.1 hosts then 2TB LUNs could be created?

Technically, it was "fixed" in 4.0 to behave properly, where 3.5 did not.

Also, the RDM maximum size seems to still be 2TB-512 byte in 4.1. What is the reason for this kind of limit? If the guest OS could accept larger disks than 2TB, should that not be enough?

The limit is around the fact that VMware ESX and ESXi uses the SCSI-2 standard. This is documented in kb 3371739, and interestingly enough seems to support that the config max document may have a mistake with regard to LUN size. In one of the sessions at VMworld, someone asked when this was going to be addressed. The reply was "you will be happy soon," so I take that mean that this issue is being worked on.

Good Luck!

[rickardnobel]

Hello, and thank you for your reply.

Interesting find. I wonder if that is a mistake? I don't recall seeing anything in the release notes about a change there.

I agree, I have not seen this in the release notes, but noticed it in the config-maximums and thought I should ask if this was known.

https://www.vmware.com/pdf/vsphere4/r41/vsp_41_config_max.pdf

Technically, it was "fixed" in 4.0 to behave properly, where 3.5 did not.

As in, it is better to have the 512 byte spare?

>In one of the sessions at VMworld, someone asked when this was going to be addressed. The reply was "you will be happy soon," so I take that mean that this issue is being worked on.

Was that for the RDM or LUN size? Both would be good of course! Was that said at the US Vmworld this year?

[vmroyale]

Technically, it was "fixed" in 4.0 to behave properly, where 3.5 did not.

As in, it is better to have the 512 byte spare?

The "Understanding the Limitation" section of kb 3371739 explains the math behind the 512. It involves a reserved value.

>In one of the sessions at VMworld, someone asked when this was going to be addressed. The reply was "you will be happy soon," so I take that mean that this issue is being worked on.

Was that for the RDM or LUN size? Both would be good of course! Was that said at the US Vmworld this year?

I will assume that it would be for both, since the current SCSI-2 limitations apply to both. The information about this changing was in the Q&A portion at the end of TA7121 - "Next Generation VM Storage Solutions with vStorage API for Array Integration (VAAI)" held in San Franciso just a couple of weeks ago.

[rickardnobel]

Even on NFS store, will the maximum virtual disk size to a VM be 2TB-512 byte?

Would a guest initiated iSCSI connection to a LUN larger than 2TB work (if the guest os could handle it)?

The information about this changing was in the Q&A portion at the end of TA7121 - "Next Generation VM Storage Solutions with vStorage API for Array Integration (VAAI)" held in San Franciso just a couple of weeks ago.

Interesting. Did you get any feeling if that possible change should come in an update for the 4.0-4.1 or appear in a later release, perhaps 4.5 or 5.0?

I will attend Vmworld in Copenhagen in october, so I should perhaps visit that session.

[vmroyale · Accepted solution]

Even on NFS store, will the maximum virtual disk size to a VM be 2TB-512 byte?

Yes, this is a maximum specificed for the virtual machine.

Would a guest initiated iSCSI connection to a LUN larger than 2TB work (if the guest os could handle it)?

Yes, this is one way around the current 2 TB - 512 byte limitation.

The information about this changing was in the Q&A portion at the end of TA7121 - "Next Generation VM Storage Solutions with vStorage API for Array Integration (VAAI)" held in San Franciso just a couple of weeks ago.

Interesting. Did you get any feeling if that possible change should come in an update for the 4.0-4.1 or appear in a later release, perhaps 4.5 or 5.0?

I will attend Vmworld in Copenhagen in october, so I should perhaps visit that session.

You know, I didn't really get a strong feeling either way about it. My gut seems to tell me that it will be in the next major product release. If my memory serves me well, one of the presenters in that session was one of the engineers that originally worked making VMFS. That session was really good, so I would recommend it either way.

[rickardnobel]

>That session was really good, so I would recommend it either way.

Thanks, I shall make sure I visit it.

It would be interesting to get some confirmation if there is a misprint in the 4.1 max-config documents. I wonder if any Vmware employee can tell if there is any change of the max LUN size, and if not the document should probably be changed.