Distributed Filesystems

Accessing remote files

remote access model
- Any read/write uses an RPC to push or pull data, which always stays on the server
upload/download model
- Files are moved to the client and then back to the server
Network file system (NFS)
- Unix-like systems rely on NFS. E.g. your home directory on ecelinux is mounted remotely using NFSv4.
- "Leases" (delegates) the file to the client, but can recall the delegation
- Uses RPCs internally
- Server decides which portion to export, to only give a window of visibility to clients

Each file might be on one server, or each could be striped across multiple servers
- Striping usually allows higher throughput for large requests issued one at a time
- Many small requests issued in parallel, one file per server is ok

Unix semantics: In a centralized setting, there is strict time ordering, so applications always read their own writes
Session semantics: Local version of file is only propagated when the file is closed, and the last client to close the file sets the final version.
NFS v4 implements session semantics and byte range file locking
- byte range file locking is used to prevent inconsistencies
- Requests to read a file will stall until a block is released
Immutable files
- no updates possible
- delete/replace is possible
- in reality (GFS, HDFS), append is also supported
Transactions are a thing too