1. Other File Systems: LFS, NFS, and AFS

2. Goals for Today Discuss specific file systems
both local and remote
Log-structured file system (LFS)
Distributed file systems (DFS)
Network file system (NFS)
Andrew file system (AFS)

3. Log-Structured File Systems
The trend: CPUs are faster, RAM & caches are bigger
So, a lot of reads do not require disk access
Most disk accesses are writes  pre-fetching not very useful
Worse, most writes are small  10 ms overhead for 50 µs write
Example: to create a new file:
i-node of directory needs to be written
Directory block needs to be written
i-node for the file has to be written
Need to write the file
Delaying these writes could hamper consistency
Solution: LFS to utilize full disk bandwidth

4. LFS Basic Idea Structure the disk a log
Periodically, all pending writes buffered in memory are collected in a single segment
The entire segment is written contiguously at end of the log
Segment may contain i-nodes, directory entries, data
Start of each segment has a summary
If segment around 1 MB, then full disk bandwidth can be utilized
Note, i-nodes are now scattered on disk
Maintain i-node map (entry i points to i-node i on disk)
Part of it is cached, reducing the delay in accessing i-node
This description works great for disks of infinite size

5. LFS vs. UFS inode directory data inode map Unix File System Log
Blocks written to
create two 1-block
files: dir1/file1 and
dir2/file2, in UFS and
LFS
Log
Log-Structured
File System
file1
file2

6. LFS Cleaning
Finite disk space implies that the disk is eventually full
Fortunately, some segments have stale information
A file overwrite causes i-node to point to new blocks
Old ones still occupy space
Solution: LFS Cleaner thread compacts the log
Read segment summary, and see if contents are current
File blocks, i-nodes, etc.
If not, the segment is marked free, and cleaner moves forward
Else, cleaner writes content into new segment at end of the log
The segment is marked as free!
Disk is a circular buffer, writer adds contents to the front, cleaner cleans content from the back

7. Distributed File Systems
Goal: view a distributed system as a file system
Storage is distributed
Web tries to make world a collection of hyperlinked documents
Issues not common to usual file systems
Naming transparency
Load balancing
Scalability
Location and network transparency
Fault tolerance
We will look at some of these today

8. Transfer Model Upload/download Model: Remote Access Model:
Client downloads file, works on it, and writes it back on server
Simple and good performance
Remote Access Model:
File only on server; client sends commands to get work done

9. Naming transparency
Naming is a mapping from logical to physical objects
Ideally client interface should be transparent
Not distinguish between remote and local files
/machine/path or mounting remote FS in local hierarchy are not transparent
A transparent DFS hides the location of files in system
2 forms of transparency:
Location transparency: path gives no hint of file location
/server1/dir1/dir2/x tells x is on server1, but not where server1 is
Location independence: move files without changing names
Separate naming hierarchy from storage devices hierarchy

10. File Sharing Semantics
Sequential consistency: reads see previous writes
Ordering on all system calls seen by all processors
Maintained in single processor systems
Can be achieved in DFS with one file server and no caching

11. Caching Keep repeatedly accessed blocks in cache How it works:
Improves performance of further accesses
How it works:
If needed block not in cache, it is fetched and cached
Accesses performed on local copy
One master file copy on server, other copies distributed in DFS
Cache consistency problem: how to keep cached copy consistent with master file copy
Where to cache?
Disk: Pros: more reliable, data present locally on recovery
Memory: Pros: diskless workstations, quicker data access,
Servers maintain cache in memory

12. File Sharing Semantics
Other approaches:
Write through caches:
immediately propagate changes in cache files to server
Reliable but poor performance
Delayed write:
Writes are not propagated immediately, probably on file close
Session semantics (AFS): write file back on close
Alternative (NFS): scan cache periodically and flush modified blocks
Better performance but poor reliability
File Locking:
The upload/download model locks a downloaded file
Other processes wait for file lock to be released

13. Network File System (NFS)
Developed by Sun Microsystems in 1984
Used to join FSes on multiple computers as one logical whole
Used commonly today with UNIX systems
Assumptions
Allows arbitrary collection of users to share a file system
Clients and servers might be on different LANs
Machines can be clients and servers at the same time
Architecture:
A server exports one or more of its directories to remote clients
Clients access exported directories by mounting them
The contents are then accessed as if they were local

14. Example

15. NFS Mount Protocol
Client sends path name to server with request to mount
Not required to specify where to mount
If path is legal and exported, server returns file handle
Contains FS type, disk, i-node number of directory, security info
Subsequent accesses from client use file handle
Mount can be either at boot or automount
Using automount, directories are not mounted during boot
OS sends a message to servers on first remote file access
Automount is helpful since remote dir might not be used at all
Mount only affects the client view!

16. NFS Protocol
Supports directory and file access via remote procedure calls (RPCs)
All UNIX system calls supported other than open & close
Open and close are intentionally not supported
For a read, client sends lookup message to server
Server looks up file and returns handle
Unlike open, lookup does not copy info in internal system tables
Subsequently, read contains file handle, offset and num bytes
Each message is self-contained
Pros: server is stateless, i.e. no state about open files
Cons: Locking is difficult, no concurrency control

17. NFS Implementation Three main layers: System call layer:
Handles calls like open, read and close
Virtual File System Layer:
Maintains table with one entry (v-node) for each open file
v-nodes indicate if file is local or remote
If remote it has enough info to access them
For local files, FS and i-node are recorded
NFS Service Layer:
This lowest layer implements the NFS protocol

18. NFS Layer Structure

19. How NFS works? Mount: Open:
Sys ad calls mount program with remote dir, local dir
Mount program parses for name of NFS server
Contacts server asking for file handle for remote dir
If directory exists for remote mounting, server returns handle
Client kernel constructs v-node for remote dir
Asks NFS client code to construct r-node for file handle
Open:
Kernel realizes that file is on remotely mounted directory
Finds r-node in v-node for the directory
NFS client code then opens file, enters r-node for file in VFS, and returns file descriptor for remote node

20. Cache coherency Clients cache file attributes and data Solutions:
If two clients cache the same data, cache coherency is lost
Solutions:
Each cache block has a timer (3 sec for data, 30 sec for dir)
Entry is discarded when timer expires
On open of cached file, its last modify time on server is checked
If cached copy is old, it is discarded
Every 30 sec, cache time expires
All dirty blocks are written back to the server

21. Andrew File System (AFS)
Named after Andrew Carnegie and Andrew Mellon
Transarc Corp. and then IBM took development of AFS
In 2000 IBM made OpenAFS available as open source
Features:
Uniform name space
Location independent file sharing
Client side caching with cache consistency
Secure authentication via Kerberos
Server-side caching in form of replicas
High availability through automatic switchover of replicas
Scalability to span 5000 workstations

22. AFS Overview Based on the upload/download model
Clients download and cache files
Server keeps track of clients that cache the file
Clients upload files at end of session
Whole file caching is central idea behind AFS
Later amended to block operations
Simple, effective
AFS servers are stateful
Keep track of clients that have cached files
Recall files that have been modified

23. AFS Details Has dedicated server machines
Clients have partitioned name space:
Local name space and shared name space
Cluster of dedicated servers (Vice) present shared name space
Clients run Virtue protocol to communicate with Vice
Clients and servers are grouped into clusters
Clusters connected through the WAN
Other issues:
Scalability, client mobility, security, protection, heterogeneity

24. AFS: Shared Name Space AFS’s storage is arranged in volumes
Usually associated with files of a particular client
AFS dir entry maps vice files/dirs to a 96-bit fid
Volume number
Vnode number: index into i-node array of a volume
Uniquifier: allows reuse of vnode numbers
Fids are location transparent
File movements do not invalidate fids
Location information kept in volume-location database
Volumes migrated to balance available disk space, utilization
Volume movement is atomic; operation aborted on server crash

25. AFS: Operations and Consistency
AFS caches entire files from servers
Client interacts with servers only during open and close
OS on client intercepts calls, and passes it to Venus
Venus is a client process that caches files from servers
Venus contacts Vice only on open and close
Does not contact if file is already in the cache, and not invalidated
Reads and writes bypass Venus
Works due to callback:
Server updates state to record caching
Server notifies client before allowing another client to modify
Clients lose their callback when someone writes the file
Venus caches dirs and symbolic links for path translation

26. AFS Implementation Client cache is a local directory on UNIX FS
Venus and server processes access file directly by UNIX i-node
Venus has 2 caches, one for status & one for data
Uses LRU to keep them bounded in size

27. Summary LFS: NFS: AFS: Local file system Optimize writes
Simple distributed file system protocol. No open/close
Stateless server
Has problems with cache consistency, locking protocol
AFS:
More complicated distributed file system protocol
Stateful server
session semantics: consistency on close

28. Enjoy Spring Break!!!

29. Storage Area Networks (SANs)
New generation of architectures for managing storage in massive data centers
For example, Google is said to have 50, ,000 computers in various centers
Amazon is reaching a similar scale
A SAN system is a collection of file systems with tools to help humans administer the system

30. Examples of SAN issues Where should a file be stored
Many of these systems have an indirection mechanism so that a file can move from volume to volume
Allows files to migrate, e.g. from a slow server to a fast one or from long term storage onto an active disk system
Eco-computing: systems that seek to minimize energy in big data centers

31. Examples of SAN issues Disk-to-disk backup
Might want to do very fast automated backups
Ideally, can support this while the disk is actively in use
Easiest if two disks are next to each other
Challenge: back up entire data center in New York at site in Kentucky
US Dept of Treasury e-Cavern

32. File System Reliability
2 considerations: backups and consistency
Why backup?
Recover from disaster
Recover from stupidity
Where to backup? Tertiary storage
Tape: holds 10 or 100s of GBs, costs pennies/GB
sequential access  high random access time
Backup takes time and space

33. Backup Issues Should the entire FS be backup up?
Binaries, special I/O files usually not backed up
Do not backup unmodified files since last backup
Incremental dumps: complete per month, modified files daily
Compress data before writing to tape
How to backup an active FS?
Not acceptable to take system offline during backup hours
Security of backup media

34. Backup Strategies Physical Dump Logical Dump
Start from block 0 of disk, write all blocks in order, stop after last
Pros: Simple to implement, speed
Cons: skip directories, incremental dumps, restore some file
No point dumping unused blocks, avoiding it is a big overhead
How to dump bad blocks?
Logical Dump
Start at a directory
dump all directories and files changed since base date
Base date could be of last incremental dump, last full dump, etc.
Also dump all dirs (even unmodified) in path to a modified file

35. Logical Dumps Why dump unmodified directories?
Restore files on a fresh FS
To incrementally recover a single file
File that has
not changed

36. A Dumping Algorithm Algorithm: Mark all dirs & modified files
Unmark dirs with no mod. files
Dump dirs
Dump modified files

37. Logical Dumping Issues
Reconstruct the free block list on restore
Maintaining consistency across symbolic links
UNIX files with holes
Should never dump special files, e.g. named pipes