Operating Systems 2019W Lecture 10
The video for the lecture given on February 6, 2019 is now available.
There are a number of chapters in the textbook related to filesystems & block devices. You are not responsible for all of this material, but you may benefit from looking through the following for key concepts covered in class - particularly if you are confused about some of the concepts.
Lecture 10 ---------- What is a filesystem? - hierarchy of files? - may or may not have underlying storage - more formally, hierarchical key-value store Really, from the kernel's perspective, it is - functions that can be called to implement filesystem-related system calls This abstraction is knows as the "virtual filesystem", VFS In tutorial 4, you're making and manipulating filesystems...stored in block devices Examples: /proc proc filesystem /sys sys filesystem / ext4 filesystem /boot/efi vfat filesystem /boot ext2 filesystem ... /boot/efi/UBUNTU <-- what filesystem is this on? mounting is how we associate a subtree (directory) with a filesystem BAD IDEA mount /dev/sdc1 / (assume /dev/sdc is a USB stick) local, persistent filesystems are normally associated with block devices - the filesystem is what allows us to use a block device to store files a block device is a persistent "array" of "blocks" - blocks are fixed size (i.e. 4K) - accessed by index From a storage perspective, a filesystem is a way of storing a file hierarchy on a block device (i.e., in blocks accessed by indices) So is a filesystem just a tree? - kind of Goals for an on-disk filesystem structure - fault tolerant - reliable - high performance - takes the characteristics of the block device into account Normally, block devices (whether hard drives or SSDs) - sequential access is faster than random access High level action on an open system call * process makes open system call, e.g. open("/home/student/foo.txt", ...) * kernel figures out what filesystem /home/student/foo.txt is on (ext4 from /dev/sda2 mounted on /home) * kernel calls ext4 open function for /dev/sda2, gives it /student/foo * ext4 looks at /dev/sda2, figures out where the blocks are for /student/foo * ext4 stores this info in file structure, returns <-- what a file descriptor refers to