On 29/08/2012 9:57 PM, James Harper wrote:
Unlikely. Windows VSS is _very_ good. Firstly, it allows snapshot sets to be taken atomically across multiple volumes (think /var/db_data and /var/db_logs) ensuring true crash-consistent snapshots. If /var/db_data and /var/db_logs were on separate LVM volumes under Linux, all bets are off.
Secondly, VSS is tightly integrated with the OS. Any decent database or other application will provide a VSS writer which tells the backup application exactly how to back up the data for the database. To give an example, for Windows 2003 and older, when a VSS snapshot is taken the registry files are dumped to a different location and instructions are provided to the backup application to back the files up from there as if they were really in C:\Windows\System32\config. For an MSSQL differential backup, the backup application is told exactly which file fragments (eg offset 1234 length 456) have changed since the full backup was taken, so that only those need to be backed up. For Exchange, log truncation etc is handled automatically.
Even without fancy instructions required, the VSS snapshot process gives applications a chance to ensure application level consistency.
Sorry to sound like a Microsoft shill, but I've contributed code to the Bacula project to do all the above stuff so I can appreciate the differences.
Yes, I have to agree that VSS is seriously good, but you still need application aware programs to be sure that the backup is fully consistent. MSSQL might be fine without special consideration, but just because it is a Windows box, doesn't mean it needs to have a native database. Oh and to Russell... 6TB could easily be multiple RAID1 volumes, who knows, it could be anything -- hence why I asked and didn't ass-u-me ... ;) There is always more than one way to skin a cat, so to speak and usually quite a number of ways. Cheers A.