1. CS 540 Database Management Systems
Lecture 4: DBMS Architecture, storage

2. The advantage of RDBMS
It separates logical level (schema) from physical level (implementation).
Physical data independence
Users do not worry about how their data is stored and processes on the physical devices.
It is all SQL!
Their queries work over (almost) all RDBMS deployments.

3. Challenges in physical level
Processor: – MIPS
Main memory: around 10 Gb/ sec.
Secondary storage: higher capacity and durability
Disk random access
Seek time + rotational latency + transfer time
Seek time: 4 ms ms!
Rotational latency: 2 ms – 7 ms!
Transfer time: at most 1000 Mb/ sec
Read, write in blocks.

4. Random access versus sequential access
Disk random access : Seek time + rotational latency + transfer time.
Disk sequential access: reading blocks next to each other
No seek time or rotational latency
Much faster than random access

5. DBMS Architecture User/Web Forms/Applications/DBA query transaction
Process manager
Query Parser
Transaction Manager
Query Rewriter
Logging &
Recovery
Query Optimizer
Lock Manager
Query Executor
Files & Access Methods
Lock Tables
Buffers
Buffer Manager
Main Memory
Storage Manager
Storage

6. DBMS Architecture User/Web Forms/Applications/DBA query transaction
Process manager
Query Parser
Transaction Manager
Query Rewriter
Logging &
Recovery
Query Optimizer
Lock Manager
Query Executor
Files & Access Methods
Lock Tables
Buffers
Buffer Manager
This lecture
Main Memory
Storage Manager
Storage

7. A Design Dilemma To what extent should we reuse OS services?
Reuse as much as we can
Performance problem (inefficient)
Lack of control (incorrect crash recovery)
Replicating some OS functions (“mini OS”)
Have its own buffer pool
Directly manage record structures with files …

8. OS vs. DBMS Similarities?
What do they manage?
What do they provide?

9. OS vs. DBMS: Similarities
Purpose of an OS:
managing hardware
presenting interface abstraction to applications
DBMS is in some sense an OS?
DBMS manages data
Both as API for application development!

10. OS vs. DBMS: Related Concepts
Process Management  What DB concepts?
process synchronization
deadlock handling
Storage management  What DB concepts?
virtual memory
file system

11. OS vs. DBMS: Differences?

12. OS vs. DBMS: Differences
DBMS: Top-down to encapsulate high-level semantics!
Data
data with particular logical structures
Queries
query language with well defined operations
Transactions
transactions with ACID properties
OS: Bottom-up to present low-level hardware

13. Problems with DBMS on top of OS
Buffer pool management
File system
Process management
Consistency control
Paged virtual memory

14. Buffer Pool Management
Performance of system calls
LRU replacement
Query-aware replacement needed for performance
Circular access: 1, 2, …, n, 1, 2, ..
Prefetching
DBMS knows exactly which block is to be fetched next
Crash recovery
Need “selected force out”

15. Relations vs. File system
Data object abstraction
file: array of characters
relation: set of tuples
Physical contiguity:
large DB files want clustering of blocks
sol1: managing raw disks by DBMS
sol2: simulate by managing free spaces in DBMS
Multiple trees (access methods)
file access: directory hierarchy (user access method)
block access: inodes
tuple access: DBMS indexes
- Sol2: DBMS asks OS for large-than-needed-now chunks, and manage space within DBMS

16. Process management Reuse OS process management
One process for each user
Problem: DB processes are large
long time to switch between processes
Problem: critical sections
Processes may have to wait for a descheduled process that has locks.
n server processes that handle users’ requests
duplication of OS multi-tasking inside servers!
communication between processes:
Message passing is not efficient
Solutions: OS implements
favored processes
not forced out, relinquish the control voluntarily.
faster message passing methods.

17. Consistency control OS provides some support for locking and recovery.
OS provides lock on files
DB requires lock on smaller units like tuples
Commit point
Buffer manager ensures all changes are flushed on disk.
Buffer manager must know the inside of transactions.

18. State of the art DBMSs duplicate some OS functionalities.
OS customized for DBMS