Databases are stored persistently, meaning data remains even when the computer is turned off. For the vast majority of databases, this persistent storage is done on hard disk drives (HDDs) or increasingly solid-state drives (SSDs). While SSDs have different internal mechanics, they share the same logical storage concepts relevant to database systems.

Why Disks? • Persistence: Data survives power loss. • Capacity: Can store vast amounts of data (terabytes). • Cost-effectiveness: Cheaper per gigabyte than main memory (RAM).

1. Block (or Page): The Fundamental Unit of I/O • Imagine a disk as a large collection of numbered compartments. The smallest amount of data that a computer can read from or write to a disk in a single operation is called a block (or sometimes a page). • Blocks have a fixed size, typically ranging from 4KB (kilobytes) to 16KB. • Importance: Disk input/output (I/O), which is the process of reading data from or writing data to the disk, happens in units of blocks. Even if you only need a single piece of data (e.g., one value from one record), the entire block containing that data must be read into the computer's main memory (RAM). • Performance Bottleneck: Disk I/O is by far the slowest operation in a database system. It involves mechanical movement (for HDDs) or electrical operations (for SSDs) that are orders of magnitude slower than operations in RAM. Therefore, a primary goal of database physical design is to minimize the number of disk I/Os.

1. Record (or Tuple): A Single Row • A record (also known as a tuple in the relational model context) represents a single row of data in a database table. For example, in a Students table, one record would contain all the information for a single student (StudentID, FirstName, LastName, MajorDeptID, etc.). • Records are stored inside blocks. A single block can hold multiple records. • Fixed-Length Records: If all records in a table have the exact same size (e.g., all columns have fixed lengths like CHAR(10) or INTEGER), they are called fixed-length records. This makes it easy to calculate where each record starts within a block. • Variable-Length Records: If records can have different sizes (e.g., due to VARCHAR columns, or optional fields), they are variable-length records. Storing these efficiently within blocks is more complex, often requiring headers within the block to indicate record start/end positions.

1. File: A Collection of Records • A file in a database context is a collection of related records. Typically, all the records belonging to a single table are stored together in a single file on disk. • A file is composed of one or more blocks. As a table grows, more blocks are allocated to its file to store the new records. • The database management system (DBMS) is responsible for managing these files, including allocating and deallocating blocks, and keeping track of where records are located within these blocks.