This article describes changes in Windows operating systems that will support disk logical unit sizes larger than 2 TB.
In past editions, Windows used 32-bit block numbers in the lower storage stack. This effectively limited support for single disk devices-which might be hardware RAID sets, sometimes called virtual disks or LUNs-to being no larger than 2 terabytes (TB). To get storage unit sizes beyond 2 TB, one had to combine multiple LUNs using the Volume Manager, leading to a volume limit of 64 TB with RAID0 or spanning or 62 TB with RAID5. RAID1 was also limited to 2 TB. The use of Volume Manager sets, however, imposes a performance penalty and can make some storage management scenarios difficult or impossible.
In contrast, the file system uses a 64-bit signed byte offset. This means that the absolute file system limit is actually 254 512-byte blocks. However, NTFS reduces this, because it supports a 32-bit cluster number * 64K per cluster maximum, which equals 256 TB.
Another complicating factor was the use of Master Book Record (MBR) partition types, which can only contain up to 232 blocks. GUID Partition Tables (GPT) can support a much larger number of blocks, but Windows only supported GPT on Intel Itanium machines.
Note: Microsoft recommends that for Windows Server 2003, basic disks should use 512-byte sectors (dynamic disks will only work with 512-byte sectors). Windows Vista and later operating systems will support up to 4-KB sector sizes.
With Windows Server 2003 Service Pack 1 (SP1) and Windows XP 64-bit Edition (x64), these limits have changed.
Microsoft added support for 64-bit block numbers in the disk/class layer, using the new SCSI Commands included in the SCSI-3 Block Commands-2 command set. Microsoft also enabled GPT support for all Windows Server 2003 SP1 platforms. With this change, for example, a snapshot of a GPT partition on an Itanium-based machine can now be transported to a 32-bit machine for data mining or archiving purposes.
The new limits are as follows:
· Basic or dynamic volume size: 264 blocks = 273 bytes (too big to pronounce)
· Maximum NTFS file system size that can be realized on Windows: 256 TB
Note: Disk devices with more than 2 TB of disk space must be converted to GPT format for all of the disk space to be usable. If the device uses MBR format, the disk space beyond 2 TB will be unusable