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

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CHAPTER 5 ■ ORACLE PROCESSES 177 online redo log is used to “fix” the data files in the event of a power failure (when the instance is terminated), archived redo logs are used to “fix” data files in the event of a hard disk failure. If we lose the disk drive containing the data file, /d01/oradata/ora10g/system.dbf, we can go to our backups from last week, restore that old copy of the file, and ask the database to apply all of the archived and online redo logs generated since that backup took place. This will “catch up” that file with the rest of the data files in our database, and we can continue processing with no loss of data. ARCn typically copies online redo log files to at least two other locations (redundancy being a key to not losing data!). These other locations may be disks on the local machine or, more appropriately, at least one will be located on another machine altogether, in the event of a catastrophic failure. In many cases, these archived redo log files are copied off by some other process to some tertiary storage device, such as tape. They may also be sent to another machine to be applied to a “standby database,” a failover option offered by Oracle. We’ll discuss the processes involved in that shortly. Remaining Focused Processes Depending on the features of Oracle you are using, other focused processes may be visible. They are listed here with a brief description of their function. The processes described previously are nonnegotiable—you will have them if you have an Oracle instance running. The following processes are optional and will appear only if you make use of the specific feature. The following processes are unique to a database instance using ASM, as discussed in Chapter 3: • Automatic Storage Management Background (ASMB) process: The ASMB process runs in a database instance that is making use of ASM. It is responsible for communicating to the ASM instance that is managing the storage, providing updated statistics to the ASM instance, and providing a “heartbeat” to the ASM instance, letting it know that it is still alive and functioning. • Rebalance (RBAL) process: The RBAL process also runs in a database instance that is making use of ASM. It is responsible for processing a rebalance request (a redistribution request) as disks are added/removed to and from an ASM disk group. The following processes are found in an Oracle RAC instance. RAC is a configuration of Oracle whereby multiple instances, each running on a separate node (typically a separate physical computer) in a cluster, may mount and open a single database. It gives you the ability to have more than one instance accessing, in a full read-write fashion, a single set of database files. The primary goals of RAC are twofold: • High availability: With Oracle RAC, if one node/computer in the cluster fails due to a software, hardware, or human error, the other nodes may continue to function. The database will be accessible via the other nodes. You might lose some computing power, but you won’t lose access to the database. • Scalability: Instead of buying larger and larger machines to handle an increasing workload (known as vertical scaling), RAC allows you to add resources in the form of more machines in the cluster (known as horizontal scaling). Instead of trading in your 4 CPU machine for one that can grow to 8 or 16 CPUs, RAC gives you the option of adding another relatively inexpensive 4 CPU machine (or more than one).
178 CHAPTER 5 ■ ORACLE PROCESSES The following processes are unique to a RAC environment. You will not see them otherwise. * Lock monitor (LMON) process: The LMON process monitors all instances in a cluster to detect the failure of an instance. It then facilitates the recovery of the global locks held by the failed instance. It is also responsible for reconfiguring locks and other resources when instances leave or are added to the cluster (as they fail and come back online, or as new instances are added to the cluster in real time). • Lock manager daemon (LMD) process: The LMD process handles lock manager service requests for the global cache service (keeping the block buffers consistent between instances). It works primarily as a broker sending requests for resources to a queue that is handled by the LMSn processes. The LMD handles global deadlock detection/resolution and monitors for lock timeouts in the global environment. • Lock manager server (LMSn) process: As noted earlier, in a RAC environment, each instance of Oracle is running on a different machine in a cluster, and they all access, in a read-write fashion, the same exact set of database files. To achieve this, the SGA block buffer caches must be kept consistent with respect to each other. This is one of the main goals of the LMSn process. In earlier releases of Oracle Parallel Server (OPS), this was accomplished via a ping. That is, if a node in the cluster needed a read-consistent view of a block that was locked in exclusive mode by another node, the exchange of data was done via a disk flush (the block was pinged). This was a very expensive operation just to read data. Now, with the LMSn, this exchange is done via very fast cache-to-cache exchange over the clusters’ high-speed connection. You may have up to ten LMSn processes per instance. • Lock (LCK0) process: This process is very similar in functionality to the LMD process described earlier, but it handles requests for all global resources other than database block buffers. • Diagnosability daemon (DIAG) process: The DIAG process is used exclusively in a RAC environment. It is responsible for monitoring the overall “health” of the instance, and it captures information needed in the processing of instance failures. Utility Background Processes These background processes are totally optional, based on your need for them. They provide facilities not necessary to run the database day to day, unless you are using them yourself, such as the job queues, or are making use of a feature that uses them, such as the new Oracle 10g diagnostic capabilities. These processes will be visible in UNIX as any other background process would be—if you do a ps, you will see them. In my ps listing from the beginning of the Focused Background Processes section (reproduced in part here), you can see that I have • Job queues configured. The CJQ0 process is the job queue coordinator. • Oracle AQ configured, as evidenced by the Q000 (AQ queue process) and QMNC (AQ monitor process).
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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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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xxiii Next up are t
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