2 TB max LUN size for RDM - does 2 TB = 2048 GB or 2 TB = 2000 GB?

[Bill_Oyler]

We have a physical LUN that is exactly 2047.9 GB in size that we want to assign to a VM via RDM. According to the VMware "Configuration Maximums" document, the "Raw Device Mapping size (TB)" maximum is listed as "2 TB."

Technically, I know that 2 TB = 2048 GB, in which case we are exactly within 0.1 GB of the limit and "should" be fine. However, a lot of vendors use base 10 arithmetic instead of base 2, and mistakenly declare 2 TB = 2000 GB. If this is the case with the VMware 2 TB limit, we won't be able to make this RDM work. Based on some searches of the VMware discussion groups, I've found a few reports of people having problems with LUNs sized at 2000 GB exactly, but when they resize the LUN to 1999 GB, their problems go away. This makes me wonder if VMware in fact declares 2 TB = 2000 GB.

Does anyone know "for sure"? Since we are doing a P2V of a rather critical server, it would not be fun to have to perform all the SAN re-zoning and LUN masking, only to discover that a 2047.9 GB LUN is too large to be used as an RDM.

Thanks,

Bill

[Texiwill]

Hello,

1KB => 1024 bytes

1MB => 1024 KB

1GB => 1000MB

1TB => 1000GB

Best regards,

Edward L. Haletky

VMware Communities User Moderator

====

Author of the book 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', Copyright 2008 Pearson Education.

CIO Virtualization Blog: http://www.cio.com/blog/index/topic/168354

As well as the Virtualization Wiki at http://www.astroarch.com/wiki/index.php/Virtualization

[mj1077 · Accepted solution]

Hi,

1 Byte = 8 Bits

1 KB = 1024 Bytes

1 MB = 1024 KB

1 GB = 1024 MB

1 TB = 1024 GB

1 PB = 1024 TB

Above is per binary mostly used for processing and memory counts, but we tend to say 1 MB is 1000 KB or like that and mostly used for disk storage....Thanks

[depping]

you should be fine!

Duncan

My virtualisation blog:

If you find this information useful, please award points for "correct" or "helpful".

[GordonG]

Bill,

I know from experience that 2048 GB LUN will NOT be recognised by ESX, but 2000 GB will work. I know the documentation says that the limit for a LUN is 2TB, but they obviously don't count in binary.

I'm pretty sure that you will not be able to recognise a RAW LUN of the size you describe.

Regards,

Graeme

[TomHowarth]

Bill, VMware uses the common notation of decimal count for Terabyte level LUNs, therefore it will be 2000GB not 2048GB. if you create a 2048 GB LUN you will only be able to format 2000GB

Tom Howarth

VMware Communities User Moderator

[bobross]

Be careful here...the disk drive vendors invariably quote capacity in decimal, i.e.

1 GB = 1,000,000,000 bytes

while many OS and application vendors display capacity in base 2, i.e.

1 GB = 102410241024 bytes = 1,073,741,824 bytes

While ESX server may use the decimal method, the VMs themselves (e.g. Windows) may use the base 2 method, so the RDM LUN you think is 1TB will show up as 976GB, because the former # is base 10 while the latter is base 2.

[Bill_Oyler]

Thanks to everyone for chiming in to my question. About 1/2 of you said that ESX considers 2 TB = 2000 GB and the other half said ESX considers 2 TB = 2048 GB. I also heard from a VMware employee who confirmed that ESX considers 2 TB = 2048 GB. So, with that tie breaker, I decided to proceed using the actual LUN (which is 2047.9 GB exactly) as an RDM to the VM, and it worked PERFECTLY! No data was lost, and ESX recognized the entire 2047.9 GB LUN without any problems.

So, for everyone's future reference, VMware uses the base 2 version of 2 TB, or 2048 GB, as the limit, rather than 2000 GB.

Bill

[ferdis]

Why is maximum for VMFS and also for RDM 2TB?

[ferdis]

Hi.

Why is maximum for VMFS and also for RDM 2TB? Today it starting to be problem becouse of capacity of disks are growing hugely.

thanks.

[mcowger]

Its because VMware uses SCSI2 protocol, which is 32bit only.

--Matt

VCP, vExpert, Unix Geek

[bdowney]

I just experienced the exact same dilema with a 2TB iSCSI LUN. ESX 3.5 refused to recognize a 2048GB LUN. I shrank the LUN down to 2047GB to test Bill's theory but still had no luck. I then shrank it once more down to 2000GB and BINGO! So, in contradiction to Bill's findings, my experience proves that VMware counts in base 10 as opposed to base 2 - at least with iSCSI LUNS presented to ESX 3.5.