Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 9: SQL Server Storage and Index Structures
We will be looking into locking extensively in Chapter 14, but we will see the lockability of objects
within SQL Server discussed in passing as we look at storage.
The Extent
An extent is the basic unit of storage used to allocate space for tables and indexes. It is made up of eight
contiguous 64KB data pages.
The concept of allocating space based on extents, rather than actual space used, can be somewhat difficult
to understand for people used to operating system storage principles. The important points about
an extent include:
❑ Once an extent is full, the next record will take up not just the size of the record, but the size of a
whole new extent. Many people who are new to SQL Server get tripped up in their space estimations
in part due to the allocation of an extent at a time rather than a record at a time.
❑ By pre-allocating this space, SQL Server saves the time of allocating new space with each record.
It may seem like a waste that a whole extent is taken up just because one too many rows were added to
fit on the currently allocated extent(s), but the amount of space wasted this way is typically not that
much. Still, it can add up — particularly in a highly fragmented environment — so it’s definitely something
you should keep in mind.
The good news in taking up all this space is that SQL Server skips some of the allocation-time overhead.
Instead of worrying about allocation issues every time it writes a row, SQL Server deals with additional
space allocation only when a new extent is needed.
Don’t confuse the space that an extent is taking up with the space that a database takes up. Whatever
space is allocated to the database is what you’ll see disappear from your disk drive’s available-space number.
An extent is merely how things are, in turn, allocated within the total space reserved by the database.
The Page
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Much like an extent is a unit of allocation within the database, a page is the unit of allocation within a
specific extent. There are eight pages to every extent.
A page is the last level you reach before you are at the actual data row. Whereas the number of pages per
extent is fixed, the number of rows per page is not — that depends entirely on the size of the row, which
can vary. You can think of a page as being something of a container for both table- and index-row data.
A row is, in general, not allowed to be split between pages.
There are a number of different page types. For purposes of this book, the types we care about are:
❑ Data — Data pages are pretty self-explanatory. They are the actual data in your table, with the
exception of any BLOB data that is not defined with the text-in-row option, varchar(max) or
varbinary(max).
❑ Index — Index pages are also pretty straightforward: They hold both the non-leaf and leaf level
pages (we’ll examine what these are later in the chapter) of a non-clustered index, as well as the
non-leaf level pages of a clustered index. These index types will become much clearer as we
continue through this chapter.