Persistence

• Data stored in memory disappears on power failure or reboot
• The OS should provide a way to store data longer term

Access

• Provide a means to reference stored objects by name
• Objects could also be accessed by contents

Files

• Blend persistence and access

Common forms of persistent storage

• File - Array of bytes
• Table - Array of typed data fields
• Persistent Object - Serialized data structure from a programming context

Common access services

• Hierarchical directories mapping names to objects
• Indexes providing access based on contents

• The recent history of persistent storage has been dominated by spinning hard drives
• Solid-state drives have begun to replace spinning drives

Disk Drive

• Store data in 512 byte blobs called sectors (other sizes are possible)
• Conceptually, an array of sectors
• The processor can request that an ordered group of sectors be transferred to RAM or vice versa

Disk drive performance

• Linear reads are much faster than random reads
• Head seeking takes
• Rotational latency (waiting for the sector to arrive under the head) also adds to latency
• Reads should be queued on the drive to allow the drive to make correct performance decisions

SSDs

• Do not experience mechanical seeks or rotational latency
• Less penalty for random reads
• Generally lower latency and high bandwidth than spinning drives
• Currently more expensive per byte

Standardization

• UNIX-like systems share the same standards (POSIX)
• POSIX is implemented by Unix systems, Mac OS, and Linux

Referencing files

• Files are commonly referenced by path name
• Files are also referenced by an integer called a file descriptor

File descriptor

• Integer
• References a files
• Can be obtained using open system call

Other endpoints

• Descriptors are often used with persistent files
• They may also be used with other destination for input and output
• Examples include stdout, sockets, and hardware devices

Inherited descriptors

• All processes inherit at least 3 file descriptors
  • Standard input (0)
  • Standard output (1)
  • Standard error (2)

unistd.h

• Includes definitions for many POSIX API components

Redirection

• Standard input and output are generally the terminal
• They can be redirected elsewhere via the shell

Open

• Get a new file descriptor
• Requires filename and a mode to open a file

Modes

• O_WRONLY - Write only
• O_RDONLY - Read only
• O_RDWR - Read and write
• O_CREAT - Create file if not present
• O_TRUNC - Empty file before writing

Close

• Closes a file descriptor allowing the OS to free its memory
• All open descriptors are closed on program termination

Memory Mapped I/O

• Map a file to a processes virtual address space
• Loads and stores to memory provide random read and write access to the file

Issues with mmap

• No easy control over when updates become persistent
• There can be no write only permissions
• Implies that files size is known

Copying

• read and write can be used to copy file sections to memory
• pread and pwrite work similarly, but require a position

Sequential operation

• Some descriptions (e.g. network sockets) only allow sequential operation
• read and write can be used to copy data sequentially

Implicit position

• read and write update a stored position in a file
• lseek can be used to adjust this stored position