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content/learning-paths/cross-platform/filesystem-cache-hit/intro.md

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+---
+title: Recap of Filesystems caching and Memory
+weight: 2
+
+### FIXED, DO NOT MODIFY
+layout: learningpathall
+---
+
+## Recap of File Systems
+
+A file system is the method an operating system uses to store, organize, and manage data on storage devices like hard drives or SSDs. Linux uses a **Virtual File System (VFS)** layer to provide a uniform interface to different file system types (e.g., `ext4`, `xfs`, `btrfs`), allowing programs to interact with them in a consistent way. 
+
+Developers typically interact with the file system through system calls or standard library functions—for example, using `open()`, `read()`, and `write()` in C to access files. For instance, reading a configuration file at `/etc/myapp/config.json` involves navigating the file system hierarchy and accessing the file’s contents through these interfaces. To speed up access to such files, the operating system creates a file system cache, managed by the kernel and residing in main memory (RAM). This cache temporarily stores recently accessed file data to speed up future reads and reduce disk I/O.
+
+
+## Observing Memory Usage
+
+The operating system has to manage the memory usage for the entire system.
+
+The command `free -h` provides a snapshot of memory usage. 
+
+```output
+               total        used        free      shared  buff/cache   available
+Mem:           7.6Gi       533Mi       7.0Gi       948Ki       215Mi       7.1Gi
+Swap:             0B          0B          0B
+```
+
+The `free` command summarises the memory usage into the following columns. 
+
+- `total`: Total installed memory
+- `used`: Memory in use (excluding cache/buffers)
+- `free`: Unused memory
+- `shared`: Memory used by tmpfs and shared between processes
+- `buff/cache`: Sum of memory used by kernel buffers and cache
+- `available`: An estimate of memory available to start new apps
+
+
+### What is the posix_fadvise syscall?
+
+`posix_fadvise` is a Linux system call that allows a program to provide the kernel with hints about its expected file access patterns, such as sequential or random reads. These hints help the kernel optimize caching and I/O performance but are purely advisory—the system may choose to ignore them. It’s particularly useful for tuning performance when working with large files or when bypassing unnecessary caching.
+
+### When the posix_fadvise syscall could be of use?
+
+You should consider using `posix_fadvise` when:
+
+- You're reading or writing large files that won't fit entirely in RAM.
+- You want to avoid polluting the cache with data you won't reuse.
+- You know the access pattern ahead of time and can optimize accordingly.
+
+The syscall doesn’t guarantee behavior, but it influences how the kernel allocates caching resources.
+
+
+