As previously mentioned, for practical purposes, a database snapshot is a read-only
database. You can run SELECT queries against it, extract data from it via SSIS, and so on,
but you can??™t back it up, change any properties for the database, or perform any other
operation you could normally perform on a database.
CHAPTER 9 n DATABASE SNAPSHOTS 230
When you query against a snapshot, SQL Server checks an in-memory map of what
pages the snapshot has. If it doesn??™t find the requested page, it redirects the request to the
main database. Figure 9-2 illustrates the process.
Figure 9-2. Processing a query involving a snapshot
The process is completely seamless to the end user. The snapshot will return data
just as if it were any other database.
A snapshot file is maintained for each .mdf or .ndf file in the source database. If the
need arises to return the source database to the way it looked at that point in time, you
can simply restore the snapshot. All modified data pages will be replaced, and the transaction
log will be reset??”it is equivalent to a point-in-time restore using the transaction
log, although no full database backup, differential backup, or transaction logs are
required.
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