How the DBMS represents the database in files on disk.

<aside> 💡 Disk-based Architecture

The DBMS assumes that the primary storage location of the database is on non-volatile disk.

The DBMS's components manage the movement of data between non-volatile and volatile storage.

</aside>

<aside> 💡 Sequential vs. Random Access

Random access on non-volatile storage is usually much slower than sequential access.

DBMS will want to maximize sequential access.

• Algorithms try to reduce number of writes to random pages so that data is stored in contiguous blocks.
• Allocating multiple pages at the same time is called an extent. </aside>

<aside> 💡 Design Goals

• Allow the DBMS to manage databases that exceed the amount of memory available.
• Reading/writing to disk is expensive, so it must be managed carefully to avoid large stalls and performance degradation.
• Random access on disk is usually much slower than sequential access, so the DBMS will want to maximize sequential access. </aside>

<aside> 💡 Why not the OS

OS can do the low level jobs for the DBMS

• Page swapping may cause page faults stalls when performing multi-thread access
• Though read-only access can be solved by allowing concurrent mmap access, write accesses still have correctness and performance problem

DBMS (almost) always wants to control things itself and can do a better job than the OS.

• Flushing dirty pages to disk in the correct order.
• Specialized prefetching.
• Buffer replacement policy.
• Thread/process scheduling.

The OS is not your friend.

</aside>

1. File Storage

The DBMS stores a database as one or more files on disk typically in a proprietary format.

<aside> 💡 Storage manager → responsible for maintaining a database's files.

• Some do their own scheduling for reads and writes to improve spatial and temporal locality of pages.

It organizes the files as a collection of pages.

• Tracks data read/written to pages.
• Tracks the available space. </aside>

<aside> 💡 Database Pages

There are three different notions of "pages" in a DBMS:

• Hardware Page (usually 4KB)
• OS Page (usually 4KB)
• Database Page (512B-16KB)

A hardware page is the largest block of data that the storage device can guarantee failsafe writes.

</aside>

1.1 Heap File Organization

<aside> 💡 Heap File → is an unordered collection of pages with tuples that are stored in random order.

Need meta-data to keep track of what pages exist in multiple files and which ones have free space.

</aside>

Approaches:

1. Linked List
2. Page Directory

<aside> 💡 Page Directory

The DBMS maintains special pages that tracks the location of data pages in the database files.

The directory also records the number of free slots per page.

Must make sure that the directory pages are in sync with the data pages.

</aside>

2. Page Layout

<aside> 💡 Page Header

Every page contains a header of metadata about the page's contents.

• Page Size
• Checksum
• DBMS Version
• Transaction Visibility
• Compression Information </aside>