What is pages and extents in SQL Server?
The page is the fundamental unit of data storage in SQL Server. An extent is a collection of eight physically contiguous pages. Extents help efficiently manage pages. This guide describes the data structures that are used to manage pages and extents in all versions of SQL Server.
What are data pages in SQL Server?
The disk space allocated to a data file is logically divided into pages which is the fundamental unit of data storage in SQL Server. A database page is an 8 KB chunk of data. When you insert any data into a SQL Server database, it saves the data to a series of 8 KB pages inside the data file.
What are database pages?
A page is a basic unit of I/O operation. A page starts with a 96-byte header in which all system information (the amount of free space, the page type, the page number, and the allocation unit ID of the object that owns the page) is stored. Another part of the page is its body or data rows (8,060 bytes).
What are the types of pages in SQL Server?
So here is a good start for folks who would like to know, common page types:
- 1 – data page.
- 2 – index page.
- 3 and 4 – text pages.
- 8 – GAM page.
- 9 – SGAM page.
- 10 – IAM page.
- 11 – PFS page.
What are different part of a SQL page?
SQL servers stores the data records in data pages. Data records are rows in heap or in the leaf level of the clustered index. A data page consist of three sections. Page Header ,actual data and row offset array.
What is SQL Server page size?
In SQL Server, the page size is 8 KB. This means SQL Server databases have 128 pages per megabyte. Each page begins with a 96-byte header that is used to store system information about the page.
What is SQL NDF?
ndf extension is a secondary database file used by Microsoft SQL Server to store user data. NDF is secondary storage file because SQL server stores user specified data in primary storage file known as MDF. It is usually stored on separate disk and can spread to multiple storage devices.
What are LDF files?
The LDF file is the transaction log that allows you to perform “point in time” restores if you have system failures. The way it works is that you can restore from your last full backup and “replay” the transactions from the transaction log file.
What are the different isolation levels?
What are the different isolation levels?
| Level | Dirty Reads | Non-repeatable |
|---|---|---|
| Repeatable Read | No | No |
| Serializable | No | No |
| Snapshot | No | No |
| Read Committed snapshot | No | Yes |
What is an extent in SQL Server?
Once SQL Server has brought one or more pages into memory, it examines the page and locates the row (s) that was requested. SQL Server does another logical grouping on top of pages; it groups eight consecutive pages into a unit called an extent. Normally, an extent, like a page, is a unit of ownership.
What is the size of the data pages in SQL Server?
Everything is stored on 8K data pages (8060 bytes are technically available). 8 data pages (64K) combines into an extent. There are 2 types of extents – mixed extent which stores data pages belong to the different objects and uniform extent which stores data pages belong to the one object.
What is the difference between a page and an extent?
The page is the fundamental unit of data storage in SQL Server. An extent is a collection of eight physically contiguous pages. Extents help efficiently manage pages. This guide describes the data structures that are used to manage pages and extents in all versions of SQL Server.
How many extents can be allocated to a single data page?
There are a few special data pages types SQL Server is using to track extents allocation. Those pages are basically bitmaps – every bit handles one extent. So one page can cover 64,000 extents or almost 4Gb of data.