Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 2 ■ ARCHITECTURE OVERVIEW 55 As noted earlier, there is, in most cases, a one-to-one relationship between an instance and a database. This is probably how the confusion surrounding the terms arises. In most peoples’ experience, a database is an instance, and an instance is a database. In many test environments, however, this is not the case. On my disk, I might have five separate databases. On the test machine, at any point in time there is only one instance of Oracle running, but the database it is accessing may be different from day to day or hour to hour, depending on my needs. By simply having many different configuration files, I can mount and open any one of these databases. Here, I have one “instance” at a time but many databases, only one of which is accessible at any point in time. So now when someone talks about an instance, you’ll know they mean the processes and memory of Oracle. When they mention the database, they are talking about the physical files that hold the data. A database may be accessible from many instances, but an instance will provide access to exactly one database at a time. The SGA and Background Processes You’re probably ready now for an abstract picture of what an Oracle instance and database looks like (see Figure 2-1). Figure 2-1. Oracle instance and database Figure 2-1 is a depiction of an Oracle instance and database in their simplest form. Oracle has a large chunk of memory called the SGA where it will, for example, do the following: • Maintain many internal data structures that all processes need access to. • Cache data from disk; buffer redo data before writing it to disk. • Hold parsed SQL plans. • And so on. Oracle has a set of processes that are “attached” to this SGA, and the mechanism by which they attach differs by operating system. In a UNIX environment, they will physically attach to
56 CHAPTER 2 ■ ARCHITECTURE OVERVIEW a large shared memory segment, a chunk of memory allocated in the operating system that may be accessed by many processes concurrently (generally using shmget() and shmat()). Under Windows, these processes simply use the C call malloc() to allocate the memory, since they are really threads in one big process and hence share the same virtual memory space. Oracle will also have a set of files that the database processes/threads read and write (and Oracle processes are the only ones allowed to read or write these files). These files hold all of our table data, indexes, temporary space, redo logs, and so on. If you were to start up Oracle on a UNIX-based system and execute a ps command, you would see that many physical processes are running, with various names. You saw an example of that earlier when you observed the pmon, smon, and other processes. I cover what each of these processes are in Chapter 5, so just be aware for now that they are commonly referred to as the Oracle background processes. They are persistent processes that make up the instance, and you will see them from the time you start the instance until the time you shut it down. It is interesting to note that these are processes, not individual programs. There is only one Oracle binary executable on UNIX; it has many “personalities” depending on what it was told to do when it starts up. The same binary executable that was run to get ora_pmon_ora10g was also used to get the process ora_ckpt_ora10g. There is only one binary executable program, named simply oracle. It is just executed many times with different names. On Windows, using the pslist tool (http://www.sysinternals.com/ntw2k/freeware/ pslist.shtml), we’ll find only one process, oracle.exe. Again, on Windows there is only one binary executable (oracle.exe). Within this process, we’ll find many threads representing the Oracle background processes. Using pslist (or any of a number of tools), we can see these threads: C:\Documents and Settings\tkyte>pslist oracle PsList 1.26 - Process Information Lister Copyright (C) 1999-2004 Mark Russinovich Sysinternals - www.sysinternals.com Process information for ORACLE-N15577HE: Name Pid Pri Thd Hnd Priv CPU Time Elapsed Time oracle 1664 8 19 284 354684 0:00:05.687 0:02:42.218 Here we can see there are 19 threads (Thd in the display) contained in the single Oracle process. These threads represent what were processes on UNIX—they are the pmon, arch, lgwr, and so on bits of Oracle. We can use pslist to see more details about each: C:\Documents and Settings\tkyte>pslist -d oracle PsList 1.26 - Process Information Lister Copyright (C) 1999-2004 Mark Russinovich Sysinternals - www.sysinternals.com Thread detail for ORACLE-N15577HE: oracle 1664:
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 1 ■ ■ ■ Developing Su
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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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CHAPTER 3 ■ FILES 105 USER is "SY
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CHAPTER 4 ■ ■ ■ Memory Struct
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 5 ■ ■ ■ Oracle Proces
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CHAPTER 5 ■ ORACLE PROCESSES 157
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CHAPTER 8 ■ ■ ■ Transactions
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CHAPTER 8 ■ TRANSACTIONS 259 So,
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284 CHAPTER 9 ■ REDO AND UNDO cri
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286 CHAPTER 9 ■ REDO AND UNDO Fir
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288 CHAPTER 9 ■ REDO AND UNDO The
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292 CHAPTER 9 ■ REDO AND UNDO Wha
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294 CHAPTER 9 ■ REDO AND UNDO row
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296 CHAPTER 9 ■ REDO AND UNDO If
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298 CHAPTER 9 ■ REDO AND UNDO ops
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300 CHAPTER 9 ■ REDO AND UNDO Inv
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318 CHAPTER 9 ■ REDO AND UNDO •
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326 CHAPTER 9 ■ REDO AND UNDO wil
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328 CHAPTER 9 ■ REDO AND UNDO Thi
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332 CHAPTER 9 ■ REDO AND UNDO Whe
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336 CHAPTER 9 ■ REDO AND UNDO tou
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 423 value of
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CHAPTER 11 ■ INDEXES 425 One of t
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CHAPTER 11 ■ INDEXES 427 We then
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CHAPTER 11 ■ INDEXES 429 we ended
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 11 ■ INDEXES 435 Table 11
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CHAPTER 11 ■ INDEXES 439 an 8KB b
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CHAPTER 11 ■ INDEXES 445 Indicate
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CHAPTER 11 ■ INDEXES 447 an index
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CHAPTER 11 ■ INDEXES 449 Table 11
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CHAPTER 13 ■ ■ ■ Partitioning
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CHAPTER 13 ■ PARTITIONING 561 els
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CHAPTER 13 ■ PARTITIONING 563 BIG
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CHAPTER 13 ■ PARTITIONING 565 Enh
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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