SBS 2008 uses the inbuilt Windows Server Backup under the covers of their wizards. This backup is a block level backup that stores it’s information as a VHD file on the destination disk no matter if it’s on a USB hard drive or a remote drive share. The SBS team have placed a wizard over the top of this for configuration, but essentially under the covers, it’s just Windows Server Backup. Other vendors such as BackupAssist have also done a great job to make it easier to use the inbuilt Windows Server Backup.
When the SBS 2008 / Windows Server Backup runs for the first time, it backs up each logical drive on the server to a separate VHD file on the destination media (in SBS this will be a server connected USB hard drive). The first backup of any logical drive is a full backup where each block is effectively copied over to the VHD file. The next backup to the same destination media includes just the incremental blocks that have changed since the full backup. Subsequent backups are only of the changes since the last backup. All of these blocks/changes are stored within the VHD file for that logical disk.
Ok – so here’s the rub. VHD files are limited in size to 2TB. That means that the 2TB VHD that is your destination VHD for any given volume can only contain the base image and changes up to 2TB in total. That’s pretty much like saying you have a 2TB tape and that’s all you can fit onto it. Once the VHD is full, there is NO OPTION to backup anything more as SBS 2008 / Windows Server Backup only allows FULL VOLUME backups. There is a process that will kill older snapshots from within the target VHD, but there is no way to determine exactly what will be in any given backup in advance (ie no way you can guarantee that the backup from Monday 2 weeks ago will be on that hard drive).
In Windows Server 2008 R2 and therefore SBSv7 and SBS Aurora, things change a little. Windows Server 2008 R2 backup has both a block level engine and a file level engine in it. If the source drive is a 2TB drive then it will automatically switch to file level backup to back it up. Files will be sent into the target backup device (VHD) until that device reaches 2TB. At that point it will attempt to prune some older backups. However the destination backup device is still limited to 2TB in total.
What this all means is that if you are using the standard backup engine in Windows Server 2008, then you are limited to 2TB for your backup devices. It does not matter that you might have a 3TB Hard drive, you simply cannot backup that much data. Ok – so how do we solve this problem? The only solution is to use third party products such as StorageCraft ShadowProtect. ShadowProtect does not have this 2TB limit as it stores it’s files in it’s own file format and also compresses the data that is being backed up. If you are designing servers that have more than 2TB in a single volume then you will want to think carefully about this aspect of your design. The standard Windows or SBS backup won’t cut it at all and will leave you without a solution.
Microsoft for their part do not yet have a solution for this problem. Given they based the destination file format on the VHD format, they have not as yet released any indication of when they will have the ability to extend the VHDs beyond the 2TB limit that they currently have. You have been warned.