distsys-oct-11-2018.md

File Systems - DistSys Lecture - Oct 11, 2018

Files are a squence of bytes

File Attributes:

• permissions
• name
• owner
• timestamp

Layers

• disk (SSD nowadays)
• driver
• file system interface (vfs on Unix)
• kernel
• OS
• standard library
• application

Types of filesytems

• FAT32
• NTFS
• ext4
• HFS+
• APFS

Distributed File System

Layers

• disk (SSD nowadays)
• driver
• file system interface (vfs on Unix)
• network
• socket
• disk

A distributed file system should be:

• Fast
• Reliable
• Concurrency-safe (handle race conditions)
• maintaining State

The State keeps track of:

• file descriptors
• the file pointer: the location where a process left off in a open file
• buffer

Stateless Servers?

• Idempotent: we get the same result no matter how many times we try something

Network File System (NFS)

• UFIDs (Unique File ID) are like inodes
  • anytime a client requests a file from a server, it returns the file’s UFID

The NFS Layer maps file descriptions to UFIDs

The NFS Layer maps file descriptions to UFIDs

Reading a file

Local Linux System

fd = open("filepath")
read(fd, buf, 1024)
read(fd, buf, 1024)
close(fd) // nothing to do on server

NFS System

ufid = LOOKUP("filepath")
READ(ufid, buf, 1024, 0) // last argument is offset
READ(ufid, buf, 1024, 1)

Writing a file

Local Linux System

fd = open("filepath")
write(fd, buf, 1024)
write(fd, buf, 1024)
close(fd) // nothing to do on server

store all the write calls and when the file is closed sending the server one aggragated nfs WRITE call

NFS System

ufid = LOOKUP("filepath")
WRITE(ufid, buf, 2048) 

• What if the server is dead while we’re trying to write to the server?
  • When close(fd) is called, NFS keeps trying to write/close over and over again