Distributed file systems

Lecture 4 – Distributed Filesystems CSE 490h – Introduction to Distributed Computing, Spring 2007 Except as otherwise noted, the content of this presentation is licensed under the Creative Commons Attribution 2.5 License.

Outline Filesystems overview NFS & AFS (Andrew File System) GFS Discussion Questions

File Systems Overview System that permanently stores data Usually layered on top of a lower-level physical storage medium Divided into logical units called “files” Addressable by a filename (“foo.txt”) Usually supports hierarchical nesting (directories) A file path joins file & directory names into a relative or absolute address to identify a file (“/home/aaron/foo.txt”)

What Gets Stored User data itself is the bulk of the file system's contents Also includes meta-data on a drive-wide and per-file basis: Drive-wide: Available space Formatting info character set ... Per-file: name owner modification date physical layout...

High-Level Organization Files are organized in a “tree” structure made of nested directories One directory acts as the “root” “ links” (symlinks, shortcuts, etc) provide simple means of providing multiple access paths to one file Other file systems can be “mounted” and dropped in as sub-hierarchies (other drives, network shares)

Low-Level Organization (1/2) File data and meta-data stored separately File descriptors + meta-data stored in inodes Large tree or table at designated location on disk Tells how to look up file contents Meta-data may be replicated to increase system reliability

Low-Level Organization (2/2) “ Standard” read-write medium is a hard drive (other media: CDROM, tape, ...) Viewed as a sequential array of blocks Must address ~1 KB chunk at a time Tree structure is “flattened” into blocks Overlapping reads/writes/deletes can cause fragmentation: files are often not stored with a linear layout inodes store all block numbers related to file

Design Considerations Smaller inode size reduces amount of wasted space Larger inode size increases speed of sequential reads (may not help random access) Should the file system be faster or more reliable ? But faster at what: Large files? Small files? Lots of reading? Frequent writers, occasional readers?

Distributed Filesystems Support access to files on remote servers Must support concurrency Make varying guarantees about locking, who “wins” with concurrent writes, etc... Must gracefully handle dropped connections Can offer support for replication and local caching Different implementations sit in different places on complexity/feature scale

NFS First developed in 1980s by Sun Presented with standard UNIX FS interface Network drives are mounted into local directory hierarchy Your home directory on attu is NFS-driven Type 'mount' some time at the prompt if curious

NFS Protocol Initially completely stateless Operated over UDP; did not use TCP streams File locking, etc, implemented in higher-level protocols Modern implementations use TCP/IP & stateful protocols

Server-side Implementation NFS defines a virtual file system Does not actually manage local disk layout on server Server instantiates NFS volume on top of local file system Local hard drives managed by concrete file systems (EXT, ReiserFS, ...) Other networked FS's mounted in by...?

NFS Locking NFS v4 supports stateful locking of files Clients inform server of intent to lock Server can notify clients of outstanding lock requests Locking is lease-based: clients must continually renew locks before a timeout Loss of contact with server abandons locks

NFS Client Caching NFS Clients are allowed to cache copies of remote files for subsequent accesses Supports close-to-open cache consistency When client A closes a file, its contents are synchronized with the master, and timestamp is changed When client B opens the file, it checks that local timestamp agrees with server timestamp. If not, it discards local copy. Concurrent reader/writers must use flags to disable caching

NFS: Tradeoffs NFS Volume managed by single server Higher load on central server Simplifies coherency protocols Full POSIX system means it “drops in” very easily, but isn’t “great” for any specific need

AFS (The Andrew File System) Developed at Carnegie Mellon Strong security, high scalability Supports 50,000+ clients at enterprise level

Security in AFS Uses Kerberos authentication Supports richer set of access control bits than UNIX Separate “administer”, “delete” bits Allows application-specific bits

Local Caching File reads/writes operate on locally cached copy Local copy sent back to master when file is closed Open local copies are notified of external updates through callbacks

Local Caching - Tradeoffs Shared database files do not work well on this system Does not support write-through to shared medium

Replication AFS allows read-only copies of filesystem volumes Copies are guaranteed to be atomic checkpoints of entire FS at time of read-only copy generation Modifying data requires access to the sole r/w volume Changes do not propagate to read-only copies

AFS Conclusions Not quite POSIX Stronger security/permissions No file write-through High availability through replicas, local caching Not appropriate for all file types

The Google File System Sanjay Ghemawat, Howard Gobioff, and Shun-Tak Leung SOSP 2003 (These slides by Alex Moshchuk)

Motivation Google needed a good distributed file system Redundant storage of massive amounts of data on cheap and unreliable computers Why not use an existing file system? Google’s problems are different from anyone else’s Different workload and design priorities GFS is designed for Google apps and workloads Google apps are designed for GFS

Assumptions High component failure rates Inexpensive commodity components fail all the time “ Modest” number of HUGE files Just a few million Each is 100MB or larger; multi-GB files typical Files are write-once, mostly appended to Perhaps concurrently Large streaming reads High sustained throughput favored over low latency

GFS Design Decisions Files stored as chunks Fixed size (64MB) Reliability through replication Each chunk replicated across 3+ chunkservers Single master to coordinate access, keep metadata Simple centralized management No data caching Little benefit due to large data sets, streaming reads Familiar interface, but customize the API Simplify the problem; focus on Google apps Add snapshot and record append operations

GFS Architecture Single master Mutiple chunkservers … Can anyone see a potential weakness in this design?

Single master From distributed systems we know this is a: Single point of failure Scalability bottleneck GFS solutions: Shadow masters Minimize master involvement never move data through it, use only for metadata and cache metadata at clients large chunk size master delegates authority to primary replicas in data mutations (chunk leases) Simple, and good enough!

Metadata (1/2) Global metadata is stored on the master File and chunk namespaces Mapping from files to chunks Locations of each chunk’s replicas All in memory (64 bytes / chunk) Fast Easily accessible

Metadata (2/2) Master has an operation log for persistent logging of critical metadata updates persistent on local disk replicated checkpoints for faster recovery

Mutations Mutation = write or append must be done for all replicas Goal: minimize master involvement Lease mechanism: master picks one replica as primary; gives it a “lease” for mutations primary defines a serial order of mutations all replicas follow this order Data flow decoupled from control flow

Atomic record append Client specifies data GFS appends it to the file atomically at least once GFS picks the offset works for concurrent writers Used heavily by Google apps e.g., for files that serve as multiple-producer/single-consumer queues

Relaxed consistency model (1/2) “ Consistent” = all replicas have the same value “ Defined” = replica reflects the mutation, consistent Some properties: concurrent writes leave region consistent, but possibly undefined failed writes leave the region inconsistent Some work has moved into the applications: e.g., self-validating, self-identifying records

Relaxed consistency model (2/2) Simple, efficient Google apps can live with it what about other apps? Namespace updates atomic and serializable

Master’s responsibilities (1/2) Metadata storage Namespace management/locking Periodic communication with chunkservers give instructions, collect state, track cluster health Chunk creation, re-replication, rebalancing balance space utilization and access speed spread replicas across racks to reduce correlated failures re-replicate data if redundancy falls below threshold rebalance data to smooth out storage and request load

Master’s responsibilities (2/2) Garbage Collection simpler, more reliable than traditional file delete master logs the deletion, renames the file to a hidden name lazily garbage collects hidden files Stale replica deletion detect “stale” replicas using chunk version numbers

Fault Tolerance High availability fast recovery master and chunkservers restartable in a few seconds chunk replication default: 3 replicas. shadow masters Data integrity checksum every 64KB block in each chunk

Deployment in Google Many GFS clusters hundreds/thousands of storage nodes each Managing petabytes of data GFS is under BigTable, etc.

Conclusion GFS demonstrates how to support large-scale processing workloads on commodity hardware design to tolerate frequent component failures optimize for huge files that are mostly appended and read feel free to relax and extend FS interface as required go for simple solutions (e.g., single master) GFS has met Google’s storage needs… it must be good!

Discussion How many sys-admins does it take to run a system like this? much of management is built in Is GFS useful as a general-purpose commercial product? small write performance not good enough? relaxed consistency model

Distributed file systems

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Distributed file systems