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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1
Storing Data: Disks and Files
Chapter 7
“Yea, from the table of my memory I’ll wipe away all trivial fond records.” -- Shakespeare, Hamlet
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2
Disks and Files
DBMS stores information on (“hard”) disks.
This has major implications for DBMS design!
READ: transfer data from disk to main memory (RAM).
WRITE: transfer data from RAM to disk.
Both are high-cost operations, relative to in-memory
operations, so must be planned carefully!
Why Not Store Everything in Main Memory?
Costs too much. $1000 will buy you either
128MB of RAM or 7.5GB of disk today.
Main memory is volatile. We want data to be
saved between runs. (Obviously!)
Typical storage hierarchy:
Main memory (RAM) for currently used data.
Disk for the main database (secondary storage).
Tapes for archiving older versions of the data
(tertiary storage).
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4
Disks
Secondary storage device of choice.
Main advantage over tapes: random access vs.
sequential.
Data is stored and retrieved in units called
disk blocks or pages.
Unlike RAM, time to retrieve a disk page
varies depending upon location on disk.
Therefore, relative placement of pages on disk has
major impact on DBMS performance!
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5
Components of a Disk
Platters
The platters spin (say, 90rps).
Spindle
The arm assembly is
moved in or out to position
a head on a desired track.
Tracks under heads make
a cylinder (imaginary!).
Disk head
Arm movement
Arm assembly
Only one head
reads/writes at any
one time.
Tracks
Sector
Block size is a multiple
of sector size (which is fixed).
Accessing a Disk Page
Time to access (read/write) a disk block:
seek time (moving arms to position disk head on track)
rotational delay (waiting for block to rotate under head)
transfer time (actually moving data to/from disk surface)
Seek time and rotational delay dominate.
Seek time varies from about 1 to 20msec
Rotational delay varies from 0 to 10msec
Transfer rate is about 1msec per 4KB page
Key to lower I/O cost: reduce seek/rotation
delays! Hardware vs. software solutions?
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 10
RAID Levels (Contd.)
Level 3: Bit-Interleaved Parity
Striping Unit: One bit. One check disk.
Each read and write request involves all disks; disk
array can process one request at a time.
Level 4: Block-Interleaved Parity
Striping Unit: One disk block. One check disk.
Parallel reads possible for small requests, large
requests can utilize full bandwidth
Writes involve modified block and check disk
Level 5: Block-Interleaved Distributed Parity
Similar to RAID Level 4, but parity blocks are
distributed over all disks
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 11
Disk Space Management
Lowest layer of DBMS software manages space
on disk.
Higher levels call upon this layer to:
allocate/de-allocate a page
read/write a page
Request for a sequence of pages must be satisfied
by allocating the pages sequentially on disk!
Higher levels don’t need to know how this is
done, or how free space is managed.
Buffer Management in a DBMS
Data must be in RAM for DBMS to operate on it!
Table of <frame# , pageid> pairs is maintained.
DB
MAIN MEMORY
DISK
disk page
free frame
Page Requests from Higher Levels
BUFFER POOL
choice of frame dictated by replacement policy
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 13
When a Page is Requested ...
If requested page is not in pool:
Choose a frame for replacement
If frame is dirty, write it to disk
Read requested page into chosen frame
Pin the page and return its address.
* If requests can be predicted (e.g., sequential scans)
pages can be pre-fetched several pages at a time!
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14
More on Buffer Management
Requestor of page must unpin it, and indicate
whether page has been modified:
dirty bit is used for this.
Page in pool may be requested many times,
a pin count is used. A page is a candidate for
replacement iff pin count = 0.
CC & recovery may entail additional I/O
when a frame is chosen for replacement.
( Write-Ahead Log protocol; more later.)
Buffer Replacement Policy
Frame is chosen for replacement by a
replacement policy:
Least-recently-used (LRU), Clock, MRU etc.
Policy can have big impact on # of I/O’s;
depends on the access pattern.
Sequential flooding : Nasty situation caused by
LRU + repeated sequential scans.
# buffer frames < # pages in file means each page
request causes an I/O. MRU much better in this
situation (but not in all situations, of course).
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 19
Page Formats: Fixed Length Records
* Record id = <page id, slot # >. In first
alternative, moving records for free space
management changes rid; may not be acceptable.
Slot 1 Slot 2
Slot N
N... 0 1 M
M ... 3 2 1
PACKED UNPACKED, BITMAP
Slot 1 Slot 2
Slot N
Free Space
Slot M 1 1
number of records
number of slots
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 20
Page Formats: Variable Length Records
* Can move records on page without changing rid;
so, attractive for fixed-length records too.
Page i
Rid = (i,N)
Rid = (i,2)
Rid = (i,1)
Pointer to start of free space SLOT DIRECTORY
N... 2 1
20 16 24 N
# slots
Files of Records
Page or block is OK when doing I/O, but
higher levels of DBMS operate on records , and
files of records.
FILE: A collection of pages, each containing a
collection of records. Must support:
insert/delete/modify record
read a particular record (specified using record id )
scan all records (possibly with some conditions on
the records to be retrieved)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 22
Unordered (Heap) Files
Simplest file structure contains records in no
particular order.
As file grows and shrinks, disk pages are
allocated and de-allocated.
To support record level operations, we must:
keep track of the pages in a file
keep track of free space on pages
keep track of the records on a page
There are many alternatives for keeping track
of this.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 23
Heap File Implemented as a List
The header page id and Heap file name must
be stored someplace.
Each page contains 2 `pointers’ plus data.
Header Page
Data Page
Data Page
Data Page
Data Page
Data Page
Data Page Pages with Free Space
Full Pages
Heap File Using a Page Directory
The entry for a page can include the number
of free bytes on the page.
The directory is a collection of pages; linked
list implementation is just one alternative.
Much smaller than linked list of all HF pages!
Data Page 1
Data Page 2
Data Page N
Header Page
DIRECTORY
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 28
Summary (Contd.)
DBMS vs. OS File Support
DBMS needs features not found in many OS’s, e.g.,
forcing a page to disk, controlling the order of
page writes to disk, files spanning disks, ability to
control pre-fetching and page replacement policy
based on predictable access patterns, etc.
Variable length record format with field offset
directory offers support for direct access to
i’th field and null values.
Slotted page format supports variable length
records and allows records to move on page.
Summary (Contd.)
File layer keeps track of pages in a file, and
supports abstraction of a collection of records.
Pages with free space identified using linked list
or directory structure (similar to how pages in file
are kept track of).
Indexes support efficient retrieval of records
based on the values in some fields.
Catalog relations store information about
relations, indexes and views. ( Information that
is common to all records in a given collection. )
