Serverless Network File Systems

1. Serverless Network File Systems

2. Network File Systems • Allow sharing among independent file systems in a transparent manner • Mounting a remote directory in NFS • Use remote procedure calls • Traditional Network File Systems, like NFS, use a central server to provide the file system services. • This work present an alternative, serverless network file system called xFS.

3. NFS Requirements / Metrics • Performance / Speed • Availability • Scalability • Fault Tolerance / Recovery

4. Limitations of Central Server Systems • All read misses and disk writes go to the server – Performance Bottleneck • Not scalable – Too many clients can hurt performance. • Expensive to increase server hardware or add servers. • Require server replication for high availability – increases cost and complexity. Also, latency to duplicate the data.

5. Serverless Network File Systems • Increased Performance - distributes control processing and data storage among cooperating workstations. • Scales easily to simplify system management. • Fault tolerance through distributed RAID and Log structured file system. • Migrates responsibility of failed components to other workstations.

6. Background • RAID – Write portion of data to each disk. • High performance (parallel accesses) • Availability • xFS uses RAID striping for files across a stripe group. • Small writes hurt performance – must do parity update. • Log-structured File System (LFS) • Append-only file system. • Leaves holes – need cleanup. • Quick writes can be delayed to help RAID • Helps recovery (checkpoints on disk)

7. Background (con’t) • Multiprocessor Cache Consistency • Statically divide physical memory evenly among processors. • Each processor manages the cache consistency state for its own physical memory. • xFS does this for files. The node storing the files keeps up with consistency. • In xFS it is dynamic – files can be managed by different nodes.

8. Goals of xFS • Provide a scalable way to subset storage servers into groups to provide efficient storage. • Scalable, distributed metadata and cache consistency management. • Flexibility to dynamically reconfigure responsibilities after failures.

9. System entities • Clients – want to access data in the system • Storage Servers – store the system’s files • Metadata Managers – hold cache consistency state and disk location metadata. • Cleaners – clean up the LFS after writes • Entities may lie on the same system or on different systems.

10. Serverless File Service • “Anything, Anywhere” – all data and metadata can be located on and move to any node in the system. • File access is faster because they are distributed across multiple workstations! • How does the system locate the data? • Key maps: manager map, imap, file directories, stripe group maps.

11. Manager Map • Table indicating which machines manage which file indices • File indices listed in the parent directory file. • Globally Replicated • Updated dynamically • On machine failure or reconfiguration of file managers • Can work as a load balancing mechanism. • Not yet implemented, but a possibility

12. Imap • Imaps are held by a file’s manager • Maps a file’s index number to the disk address of the index node (inode). • The index node gives the file offset and pointers to each data block. • Similar to standard OS implementation

13. RAID Stripe Groups • Better to stripe files over a group of servers instead of all servers in the system. • Improves availability – Each group stores its own parity. Allows recovery from multiple failures. • Stripe Group Map – tells which nodes are a member of the group. • Must reference this map before reading or writing data to the file system.

14. Cache Consistency • Token-based scheme • Client must request and acquire write ownership from the file’s manager. • Manager invalidates other cached copies

15. Cache Consistency (con’t) • Client keeps write ownership until another client requests it. It then must flush the changes to the disk. • xFS guarantees that the up-to-date copy is given to the node requesting the data. • Traditional network file systems do not always guarantee this.

16. Management Distribution Policies • xFS tries to assign files used by a client to a manager co-located on that machine. • When a client creates a file, xFS assigns the manager on that machine to the file. • Improves locality • Reduces network hops to satisfy requests - 40%

17. Reconfiguration • Not yet implemented in this version… • When system detects configuration change, a global consensus algorithm is envoked. • Leader is chosen to run the algorithm given a list of active nodes. • Generates a new manager map and distributes it across the nodes.

18. Security in xFS • Only appropriate in a restricted environment • Machines cooperating over a fast network • Must trust one another’s kernels to enforce security