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MSSQL snapshot copy-on-write很棒的詮釋

copy-on-write

 

The snapshot files contain only the data that has changed from the source. For every file, SQL Server creates a bitmap that’s kept in cache, with a bit for each page of the file indicating whether that page has been copied to the snapshot. Every time a page in the source is updated, SQL Server checks the bitmap for the file to see whether the page has already been copied and, if it hasn’t, it’s copied at that time. This operation is called a copy-on-write operation.