The filesystem is the only thing that can store anything across power offs.

disk

Unlike memory, its “sector-addressable”: 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.

filters are how beliefs are updated from observation

discrete state filter

\begin{equation} b’(s’) = P(s’|b,a,o) \end{equation}

\(b’\) is what state we think we are in next, and its a probability distribution over all states, calculated given from \(b,a,o\) our current belief about our state, our action, and our observation.

We can perform this belief update by performing Bayes Theorem over \(o\):

\begin{align} b’(s’) &= P(s’|b,a,o) \\ &\propto P(o|b,a,s’) P(s’ | b,a) \end{align}