1. EECS 262a Advanced Topics in Computer Systems Lecture 20 GFS/BigTable November 1st, 2018
John Kubiatowicz
(with slides from Ion Stoica and Ali Ghodsi)
Electrical Engineering and Computer Sciences
University of California, Berkeley
CS252 S05

2. Today’s Papers Thoughts?
The Google File System. Sanjay Ghemawat, Howard Gobioff, and Shun-Tak Leung, Symposium on Operating Systems Design and Implementation (OSDI), 2003
Bigtable: a distributed storage system for structured data. Appears in Proceedings of the 7th Conference on USENIX Symposium on Operating Systems Design and Implementation (OSDI), 2006
Thoughts?

3. Distributed File Systems before GFS
Pros/Cons of these approaches?

4. What is Different about GFS environment?
Component failures are norm rather than exception, why?
File system consists of 100s/1,000s commodity storage servers
Comparable number of clients
Much bigger files, how big?
GBs file sizes common
TBs data sizes: hard to manipulate billions of KB size blocks
Really high access rate for files
Streaming access patterns
Apps want to process data in bulk at a high rate
Few have stringent response time requirements on individual reads/writes

5. What is Different about GFS environment?
Common mode: Files are mutated by appending new data, and not overwriting
Random writes practically non-existent
Once written, files are only read sequentially
Own both applications and file system  can effectively co-designing applications and file system APIs
Relaxed consistency model
Atomic append operation: allow multiple clients to append data to a file with no extra synchronization between them

6. Assumptions and Design Requirements (1/2)
Should monitor, detect, tolerate, and recover from failures
Store a modest number of large files (millions of 100s MB files)
Small files supported, but no need to be efficient
Workload
Large sequential reads
Small reads supported, but ok to batch them
Large, sequential writes that append data to files
Small random writes supported but no need to be efficient

7. Assumptions and Design Requirements (2/2)
Implement well-defined semantics for concurrent appends
Producer-consumer model
Support hundreds of producers concurrently appending to a file
Provide atomicity with minimal synchronization overhead
Support consumers reading the file as it is written
High sustained bandwidth more important than low latency

8. API
Not POSIX, but..
Support usual ops: create, delete, open, close, read, and write
Support snapshot and record append operations
Concurrent Append is correct without additional locking

9. Architecture Single Master (why?)
READ. Filename + Offset  Chunk Index, ask server for chunk handle and location
How can the MASTER SCALE? Caching, only metadata,

10. Architecture
64MB chunks identified by unique 64 bit identifier

11. Discussion 64MB chunks: pros and cons? How do they deal with hotspots?
Replication and migration
How do they achieve master fault tolerance?
Logging of state changes on multiple servers
Rapid restart: no difference between normal and exceptional restart
How does master achieve high throughput, perform load balancing, etc?
• 64MB: less master util, less traffic to master, metadata in memory
• Hotspots, replication factor app startup time
• Shadow masters, do everything but write, quick startup seconds
• Data vs Control plane, spreads chunks

12. Consistency Model Mutation: write/append operation to a chunk
Use lease mechanism to ensure consistency:
Master grants a chunk lease to one of replicas (primary)
Primary picks a serial order for all mutations to the chunk
All replicas follow this order when applying mutations
Global mutation order defined first by
lease grant order chosen by the master
serial numbers assigned by the primary within lease
If master doesn’t hear from primary, it grant lease to another replica after lease expires
• Advantages? Offload master.
• Separate data control

13. Write Control and data Flow
Optimize use of network bandwidth
Control and data separated
Network topology taken info account for forwarding
Cut-through routing (data forwarding started immediately rather than waiting for complete reception)
Primary chooses update order—independent of data transmission

14. Atomic Writes Appends Client specifies only the data
GFS picks offset to append data and returns it to client
Primary checks if appending record will exceed chunk max size
If yes
Pad chunk to maximum size
Tell client to try on a new chunk
Chunk replicas may not be exactly the same
Append semantics are “at least once”
Application needs to be able to disregard duplicates
• Advantage? Multiple writes without sync or locks

15. Master Operation Namespace locking management Replica Placement
Each master operation acquires a set of locks
Locks are acquired in a consistent total order to prevent deadlock
Replica Placement
Spread chunk replicas across racks to maximize availability, reliability, and network bandwidth
• Why namespace locking? Locking scheme allows concurrent mutations in same directory
• Replas across racks? Limited bandwidth across them.

16. Others Chunk creation Rebalancing Garbage collection
Equalize disk utilization
Limit the number of creations on same chunk server
Spread replicas across racks
Rebalancing
Move replica for better disk space and load balancing.
Remove replicas on chunk servers with below average free space
Garbage collection
Lazy deletion: default keeps data around for 3 days
Let chunk servers report which chunks they have: tolerant to a variety of faults/inconsistencies
Master knows which chunks are live/up-to-date: tells chunk servers which of reported chunks to delete
• How to ensure not lots of padded empty files? ¼
• Creations means lots of writes, so spread those

17. Summary GFS meets Google storage requirements
Optimized for given workload
Simple architecture: highly scalable, fault tolerant
Why is this paper so highly cited?
Changed all DFS assumptions on its head
Thanks for new application assumptions at Google

18. Is this a good paper? What were the authors’ goals?
What about the evaluation/metrics?
Did they convince you that this was a good system/approach?
Were there any red-flags?
What mistakes did they make?
Does the system/approach meet the “Test of Time” challenge?
How would you review this paper today?

19. BREAK

20. BigTable Distributed storage system for managing structured data
Designed to scale to a very large size
Petabytes of data across thousands of servers
Hugely successful within Google – used for many Google projects
Web indexing, Personalized Search, Google Earth, Google Analytics, Google Finance, …
Highly-available, reliable, flexible, high-performance solution for all of Google’s products
Offshoots/followons:
Spanner: Time-based consistency
LevelDB: Open source incorporating aspects of Big Table

21. Motivation Lots of (semi-)structured data at Google Big Data scale
URLs:
Contents, crawl metadata, links, anchors, pagerank, …
Per-user data:
User preference settings, recent queries/search results, …
Geographic locations:
Physical entities (shops, restaurants, etc.), roads, satellite image data, user annotations, …
Big Data scale
Billions of URLs, many versions/page (~20K/version)
Hundreds of millions of users, thousands or q/sec
100TB+ of satellite image data

22. What about a Parallel DBMS?
Data is too large scale!
Using a commercial approach would be too expensive
Building internally means system can be applied across many projects for low incremental cost
Low-level storage optimizations significantly improve performance
Difficult to do when running on a DBMS

23. Goals A general-purpose data-center storage system
Asynchronous processes continuously updating different pieces of data
Access most current data at any time
Examine changing data (e.g., multiple web page crawls)
Need to support:
Durability, high availability, and very large scale
Big or little objects
Very high read/write rates (millions of ops per second)
Ordered keys and notion of locality
Efficient scans over all or interesting subsets of data
Efficient joins of large one-to-one and one-to-many datasets

24. BigTable Distributed multi-level map Fault-tolerant, persistent
Scalable
Thousands of servers
Terabytes of in-memory data
Petabyte of disk-based data
Millions of reads/writes per second, efficient scans
Self-managing
Servers can be added/removed dynamically
Servers adjust to load imbalance

25. Building Blocks Building blocks: BigTable uses of building blocks:
Google File System (GFS): Raw storage
Scheduler: schedules jobs onto machines
Lock service: distributed lock manager
MapReduce: simplified large-scale data processing
BigTable uses of building blocks:
GFS: stores persistent data (SSTable file format for storage of data)
Scheduler: schedules jobs involved in BigTable serving
Lock service: master election, location bootstrapping
Map Reduce: often used to read/write BigTable data

26. BigTable Data Model
A big sparse sparse, distributed persistent multi-dimensional sorted map
Rows are sort order
Atomic operations on single rows
Scan rows in order
Locality by rows first
Columns: properties of the row
Variable schema: easily create new columns
Column families: groups of columns
For access control (e.g. private data)
For locality (read these columns together, with nothing else)
Harder to create new families
Multiple entries per cell using timestamps
Enables multi-version concurrency control across rows

27. Basic Data Model Multiple entries per cell using timestamps
Enables multi-version concurrency control across rows
(row, column, timestamp)  cell contents
Good match for most Google applications:
Large collection of web pages and related information
Use URLs as row keys
Various aspects of web page as column names
Store contents of web pages in the contents: column under the timestamps when they were fetched

28. Rows Row creation is implicit upon storing data
Rows ordered lexicographically
Rows close together lexicographically usually on one or a small number of machines
Reads of short row ranges are efficient and typically require communication with a small number of machines
Can exploit this property by selecting row keys so they get good locality for data access
Example:
math.gatech.edu, math.uga.edu, phys.gatech.edu, phys.uga.edu VS
edu.gatech.math, edu.gatech.phys, edu.uga.math, edu.uga.phys

29. Columns Columns have two-level name structure: Column family
family:optional_qualifier
Column family
Unit of access control
Has associated type information
Qualifier gives unbounded columns
Additional levels of indexing, if desired

30. Timestamps Used to store different versions of data in a cell
New writes default to current time, but timestamps for writes can also be set explicitly by clients
Lookup options:
“Return most recent K values”
“Return all values in timestamp range (or all values)”
Column families can be marked w/ attributes:
“Only retain most recent K values in a cell”
“Keep values until they are older than K seconds”

31. Basic Implementation
Writes go to log then to in-memory table “memtable” (key, value)
Periodically: move in memory table to disk => SSTable(s)
“Minor compaction”
Frees up memory
Reduces recovery time (less log to scan)
SSTable = immutable ordered subset of table: range of keys and subset of their columns
One locality group per SSTable (for columns)
Tablet = all of the SSTables for one key range + the memtable
Tablets get split when they get too big
SSTables can be shared after the split (immutable)
Some values may be stale (due to new writes to those keys)

32. Basic Implementation
Reads: maintain in-memory map of keys to {SSTables, memtable}
Current version is in exactly one SSTable or memtable
Reading based on timestamp requires multiple reads
May also have to read many SSTables to get all of the columns
Scan = merge-sort like merge of SSTables in order
“Merging compaction” – reduce number of SSTables
Easy since they are in sorted order
Compaction
SSTables similar to segments in LFS
Need to “clean” old SSTables to reclaim space
Also to actually delete private data
Clean by merging multiple SSTables into one new one
“Major compaction” => merge all tables

33. Locality Groups Group column families together into an SSTable
Avoid mingling data, ie page contents and page metadata
Can keep some groups all in memory
Can compress locality groups
Bloom Filters on locality groups – avoid searching SSTable
Efficient test for set membership: member(key)  true/false
False => definitely not in the set, no need for lookup
True => probably is in the set (do lookup to make sure and get value)
Generally supports adding elements, but not removing them
… but some tricks to fix this (counting)
… or just create a new set once in a while

34. Bloom Filters Basic version: Use in BigTable m bit positions
k hash functions
for insert: compute k bit locations, set them to 1
for lookup: compute k bit locations
all = 1 => return true (may be wrong)
any = 0 => return false
1% error rate ~ 10 bits/element
Good to have some a priori idea of the target set size
Use in BigTable
Avoid reading all SSTables for elements that are not present (at least mostly avoid it)
Saves many seeks

35. Three Part Implementation
Client library with the API (like DDS)
Tablet servers that serve parts of several tables
Master that tracks tables and tablet servers
Assigns tablets to tablet servers
Merges tablets
Tracks active servers and learns about splits
Clients only deal with master to create/delete tables and column family changes
Clients get data directly from servers
All Tables Are Part of One Big System
Root table points to metadata tables
Never splits => always three levels of tablets
These point to user tables

36. Many Tricky Bits SSTables work in 64k blocks
Pro: caching a block avoid seeks for reads with locality
Con: small random reads have high overhead and waste memory
Solutions?
Compression: compress 64k blocks
Big enough for some gain
Encoding based on many blocks => better than gzip
Second compression within a block
Each server handles many tablets
Merges logs into one giant log
Pro: fast and sequential
Con: complex recovery
Recover tablets independently, but their logs are mixed…
Solution in paper: sort the log first, then recover...
Long time source of bugs
Could we keep the logs separate?

37. Lessons learned
Interesting point – only implement some of the requirements, since the last is probably not needed
Many types of failure possible
Big systems need proper systems-level monitoring
Detailed RPC trace of specific requests
Active monitoring of all servers
Value simple designs

38. Is this a good paper? What were the authors’ goals?
What about the evaluation/metrics?
Did they convince you that this was a good system/approach?
Were there any red-flags?
What mistakes did they make?
Does the system/approach meet the “Test of Time” challenge?
How would you review this paper today?