Understanding the 2 TB Limit in Windows Storage

https://blogs.technet.microsoft.com/askcore/2010/02/18/understanding-the-2-tb-limit-in-windows-storage/

Understanding the 2 TB Limit in Windows Storage

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February 18, 2010 by Jeff Hughes (MSFT) // 24 Comments

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“Why can’t I have a drive bigger than 2 TB?”

This is a common question.  And as storage has gotten bigger and cheaper, I see it more and more.  So let’s take a few minutes to talk about the mysterious 2 TB limit.

There are actually three different 2 TB limitations that are commonly hit…

• Partition size

• Number of clusters

• SCSI goo

Partition Size Limitation

The partition size is pretty straight forward.  On an MBR (Master Boot Record) disk, the locations where the partition sizes are stored are only 4 bytes long.  Since this is in hexadecimal, the largest value we can stuff in there is all F’s.  So the max value would 4,294,967,295 in decimal. 

FF FF FF FFh = 4294967295d

This maximum partition size is not in bytes, it is in number of sectors.  Since currently sectors are limited to 512 bytes, the maximum size ends up being 2 TB.

4,294,967,295 sectors * 512 bytes/sectors = 2,199,023,255,040 bytes or 2TB.

Number of Clusters

The second limitation is harder to spot.  It is a limitation of NTFS.  NTFS is limited to (2^32 -1) clusters….no matter what.  The smallest cluster size possible is 512 bytes (1 sector).  So again the math leaves us at 2,199,023,255,040 or 2TB.

(2^32)-1 = (4,294,967,296)-1 = 4,294,967,295 clusters

4,294,967,295 clusters * 512 bytes/cluster = = 2,199,023,255,040 bytes or 2TB

Here is a quick reference chart to help you see the maximum NTFS size for each cluster size.

Cluster size	NTFS Max Size
512 bytes	2,199,023,255,040 (2TB)
1024 bytes	4,398,046,510,080 (4TB)
2048 bytes	8,796,093,020,160 (8TB)
4096 bytes	17,592,186,040,320 (16TB) Default cluster size
8192 bytes	35,184,372,080,640 (32TB)
16384 bytes	70,368,744,161,280 (64TB)
32768 bytes	140,737,488,322,560 (128TB)
65536 bytes	281,474,976,654,120 (256TB)

 

 

Cluster size really depends on your needs.  While 512 is fine if you just have a bunch of tiny files, it isn’t as efficient for say a volume with just SQL DBs.  Also, a tiny cluster size can adversely affect VSS.  But that is a topic for another time.

 

SCSI Goo

This is by far the hardest to understand as it requires some basic SCSI knowledge.  Microsoft Windows operating systems support two different SCSI standards when it comes to reads and writes.  There is a third but it is very old and is mostly just used on tape devices.  So let’s just forget about that one and stick to the two that are relevant.

These two standards are Read10/Write10 and Read16/Write16.  This all has to do with the way the CDB (Command Descriptor Block) is structured.

Read10/Write10 – This standard reserves bytes 2-5 to define the LBA (Logical Block Address).  Think of LBA as sector numbers….it makes it easier on your brain.  So we have 4 bytes that can define the addressable sectors.  Just like in the ‘partition size limitation’ we are back to dealing with a 4 byte number used to define all the addresses on the drive.

FF FF FF FFh = 4294967295d

And just like before, the above is just the possible number of address (number of sectors).  Multiply by the standard sector size of 512 bytes and we get…

4,294,967,295 sectors * 512 bytes/sectors = 2,199,023,255,040 bytes or 2TB.

What this all means is that when Windows is using the Read10/Write10 standard, then the biggest drive that will be supported is 2TB. 

Read16/Write16 – Sometimes called LBA64 by some vendors, this standard reserves bytes 2-9 to define the LBAs.  That would be 8 bytes, each byte being 8 bits in size.  Now here is where we start getting into some really big numbers.  The approximate size comes out to be around 8ZB (zettabytes).  Here’s a quick chart to put it in some perspective.

• 1024KB = 1MB (megabyte)

• 1024MB = 1GB (gigabyte)

• 1024GB = 1TB (terabyte)

• 1024TB = 1PB (petabyte)

• 1024PB = 1EB (exabyte)

• 1024EB = 1ZB (zettabyte)

 

So it is going to be a while before we have to worry about running into the limitation of Read16/Write16.

Exceeding the limitations

Each limitation has a way of getting around it.  Otherwise we’d still be stuck at 2TB. 

Partition size limitation – There are actually two ways around this.  The first way is to convert your disks to dynamic disks and create volume sets.  This functionality has been around since Windows 2000.  This doesn’t really allow you to increase the partition size.  What it does is give you the ability to chain your partitions together to form larger volumes.  So I could use two drives of 2TB and create a volume of roughly 4TB in size.

The second method to bypass the partition size limitation is to use a GPT (Guid Partition Table) configuration.  In Windows 2003 SP1 Microsoft introduced its implementation of the GPT.  A disk configured to be GPT rather than the MBR style would have a 32 sector partition array instead of a tiny 64 byte partition table.

NOTE:  32 sectors is equal to 16,384 bytes