---
title: "filesystem"
type: concept
related: [Contiguous Allocation, Filesystem, Linked Files, Unix V6 Filesystem, Windows Fat]
source: https://www.jemoka.com/posts/kbhfilesystem/
confidence: high
status: active
---

The filesystem is the only thing that can store anything across power offs.
disk Unlike memory, its &ldquo;sector-addressable&rdquo;: you cannot read or write individual bytes. The disk is divided into sectors, which you have to wholesale read and write.
seeking Because disks are mostly moving, reading and writing requires seeking: to wait until the platter go under the arm and read.
filesystems are designed to minimize the seek time.
functionality creating looknig up reading: sequential + random file access; access either all of a file or a part of the file editing creating folders main challenges disk space management: minimize seeks, sharing space, efficient use of disk naming: how do users name files reliability: surviving OS crashes and hardware failures protection: isolation between users, controlled sharing block a block is a group of one or more sectors, which issued to abstract away chunks of sectors.
fragmentation internal fragmentation A file can be no less than a single block of text.
external fragmentation &ldquo;no space is available even if the space in aggregate is available&rdquo;
models of storage We typically put two things into the block:
file payload data file meta-data there is a few ways to do this:
contiguous allocation linked files Windows FAT File Payload Data Unix V6 Filesystem: inode, block, Block Cache
