ECE 569
Database System Engineering
Spring 2003
Yanyong Zhang www.ece.rutgers.edu/~yyzhang
Course URL www.ece.rutgers.edu/~yyzhang/spring03
ECE569 Lecture 03-1.1
Spring 2003
Warm-up Discussion

How do we access files?

I-node, buddy system
ECE569 Lecture 03-1.2
Spring 2003
Questions to answer in this class?

If a query tries to access tuple T, how can the
system locate where T is?
ECE569 Lecture 03-1.3
Spring 2003
Media and File Management


Abstraction

Array of fixed length blocks

Size varies dynamically, space permitting
Notation

Blocks – File system objects

Pages – Units of virtual address space

Slots – Units of storage on disk

All of these items are identical in size and there is a direct
mapping from one to the other.

We will generally use them interchangeably.
ECE569 Lecture 03-1.4
Spring 2003
Data structure for Block
#define EIGHTK 8192
typedef
unsigned int FILENO;
typedef
unsigned int BLOCKID;
typedef
struct {
int
FILENO
BLOCKID
flip;
fileno;
blockno;
} BLOCKHEAD;
typedef struct
{ BLOCKHEAD
header;
char contents[EIGHTK-sizeof(header)-2];
int flop;
} BLOCK, *BLOCKP;
ECE569 Lecture 03-1.5
Spring 2003
File System API
STATUS create(filename, allocparmp)
-- create and allocate a new file
STATUS delete(filename)
-- delete a file and deallocate space for it
STATUS open(filename,ACCESSMODE,FILEID)
-- Open file in desired mode and return file handle
STATUS close(FILEID)
-- Close an open file
STATUS extend(FILEID,allocparamp)
-- extend existing file by specified amount
STATUS read(FILEID,BLOCKID,BLOCKP)
-- read contents of specified block into a buffer
STATUS readc(FILEID,BLOCKID,blockcount,BLOCKP)
-- read a certain number of block into consecutive buffers in memory.
STATUS write(FILEID,BLOCKID,BLOCKP)
-- write a buffer to the specified block on disk.
STATUS writec(FILEID,BLOCKID,blockcount,BLOCKP)
-- write a number of blocks from consecutive pages to disk.
ECE569 Lecture 03-1.6
Spring 2003
Issues in Managing Disk Space

Initial allocation: When a file is created, how many
contiguous slots should be allocated to it?

Incremental expansion: If an existing file grows
beyond the number of slots currently allocated,
how many additional contiguous blocks should be
assigned to that file?

Reorganization: When and how should the free
space on the disk be reorganized?
ECE569 Lecture 03-1.7
Spring 2003
Free Space Management
+ Bit Map
- One bit represents each block
- Easy to find contiguous free spaces for allocation
+ Free List
- Link free blocks together into a free list
- Can use all techniques for memory free-list
management, first-fit, best-fit, etc.
ECE569 Lecture 03-1.8
Spring 2003
Extent-based Allocation

Provides many of the advantages of contiguous
allocation without many of the problems

The Idea


Allocate an initial chunk that is probably big enough

If file runs out of space, allocate another chunk

Successive allocations increase in size
Characteristics

Good clustering allows efficient sequential I/O

More complex address translation than contiguous
allocation
ECE569 Lecture 03-1.9
Spring 2003
Extent-based Allocation
Why do we need extent directory?
ECE569 Lecture 03-1.10
Spring 2003
Mapping Relations to Disks
ECE569 Lecture 03-1.11
Spring 2003
Buffer Management
ECE569 Lecture 03-1.12
Spring 2003
Logic of Buffer Manager
+ Search in buffer: Check if the requested page is in the buffer. If
found, return the address F of this frame to the caller.
+ Find free frame: If the page is not in the buffer, find a frame that
holds no valid page.
+ Determine replacement victim: If no such frame exists, determine a
page that can be removed from the buffer (in order to reuse its
frame).
+ Write modified page: If replacement page has been changed, write
it.
+ Establish frame address: Denote the start address of the frame as F.
+ Determine block address: Translate the requested PAGEID P into a
FILEID and a block number. Read the block into the frame selected.
+ Return: Return the frame address F to the caller.
ECE569 Lecture 03-1.13
Spring 2003
Lost Update Anomoly
This implies that the buffer
should be shared by multiple
processes.
ECE569 Lecture 03-1.14
Spring 2003
The Need for Synchronization
ECE569 Lecture 03-1.15
Spring 2003
The Fix / Use / Unfix Protocol
+ FIX: The client requests access to a page using the
bufferfix interface.
+ USE: The client uses the page and the pointer to the
frame containing the page will remain valid.
+ UNFIX: The client explicitly waives further usage of the
frame pointer; that is, it tells the buffer manager that it
no longer wants to use that page.
ECE569 Lecture 03-1.16
Spring 2003
The Fix / Use / Unfix Protocol
page P
use
page R
fix page P
use
use
unfix page P
use
fix page R
page Q
unfix page R
use
unfix page Q
use
fix page Q
use
use
ECE569 Lecture 03-1.17
Spring 2003
Buffer Control Blocks
typedef struct {
PAGEID
pageid;
/* id of page in file*/
PAGEPTR
pageaddr;
/* base addr. in buffer*/
Int
index;
/* record within page */
Semaphore
*pagesem;
/* pointer to the sem. */
Boolean
modified;
/* caller modif. page*/
Boolean
invalid;
/* destroyed page
*/
} BUFFER_ACC_CB, *BUFFER_ACC_CBP;
ECE569 Lecture 03-1.18
Spring 2003
Buffer Structure
ECE569 Lecture 03-1.19
Spring 2003
Buffer Control Policies
+ Steal policy: When the buffer manager needs
space, it can decide to replace dirty pages.
+ No-Steal policy: Pages can be replaced only if they
are clean.
+ Force policy: At end of transaction, all modified
pages are forced to disk in a series of
synchronous write operations.
+ No-Force policy: No modified page is forced during
commit. REDO log records are written to the log.
ECE569 Lecture 03-1.20
Spring 2003