Category Archives: Online Redo Log

  1. Home / 
  2. Archive by category "Online Redo Log"
Jul 22, 2024
Database Block Buffer Cache- Memory Structures

So far, we have looked at relatively small components of the SGA. Now we are going to look at one that is potentially huge in size. The block buffer cache is where Oracle stores database blocks before writing them to disk and after reading them in from disk.

This is a crucial area of the SGA for us. Make it too small and our queries will take forever to run. Make it too big and we’ll starve other processes (e.g., we won’t leave enough room for a dedicated server to create its PGA, and we won’t even get started).

We have three places to store cached blocks from individual segments in the SGA:

•\ Default pool: The location where all segment blocks are normally cached. This is the original—and, previously, the only—buffer pool.
•\ Keep pool: An alternate buffer pool where by convention you assign segments that are accessed fairly frequently, but still get aged out of the default buffer pool due to other segments needing space.
•\ Recycle pool: An alternate buffer pool where by convention you assign large segments that you access very randomly and which would therefore cause excessive buffer flushing of many blocks from many segments. There’s no benefit to caching such segments because by the time you wanted the block again, it would have been aged out of the cache. You would separate these segments out from the segments in the default and keep pools so they would not cause those blocks to age out of the cache.

Note that in the keep and recycle pool descriptions I used the phrase “by convention.” There is nothing in place to ensure that you use either the keep pool or the recycle pool in the fashion described. In fact, the three pools manage blocks in a mostly identical fashion; they do not have radically different algorithms for aging or caching blocks.

The goal here was to give the DBA the ability to segregate segments to hot, warm, and “do not care to cache” areas. The theory was that objects in the default pool would be hot enough (i.e., used enough) to warrant staying in the cache all by themselves.

The cache would keep them in memory since they were very popular blocks. If you had some segments that were fairly popular but not really hot, these would be considered the warm blocks. These segments’ blocks could get flushed from the cache to make room for blocks you used infrequently (the “do not care to cache” blocks).

To keep these warm segments’ blocks cached, you could do one of the following:

•\ Assign these segments to the keep pool, in an attempt to let the warm blocks stay in the buffer cache longer.

•\ Assign the “do not care to cache” segments to the recycle pool, keeping the recycle pool fairly small so as to let the blocks come into the cache and leave the cache rapidly (decrease the overhead of managing them all).

Having to do one of these two things increased the management work the DBA had to perform, as there were three caches to think about, size, and assign objects to. ­Remember also that there is no sharing among them, so if the keep pool has lots of unused space, it won’t give it to the overworked default or recycle pool.

All in all, these pools were generally regarded as a very fine, low-level tuning device, only to be used after most other tuning alternatives had been looked at (if I could rewrite a query to do one-tenth the I/O rather than set up multiple buffer pools, that would be my choice).

There are up to four more optional caches, the DB_nK_CACHE_SIZE, to consider in addition to the default, keep, and recycle pools. These caches were added in support of multiple block sizes in the database. A database can have a default block size, which is the size of the blocks stored in the default, keep, or recycle pool, as well as up to four nondefault block sizes, as explained in Chapter 3.

The blocks in these buffer caches are managed in the same way as the blocks in the original default pool—there are no special algorithm changes for them either. Let’s now move on to see how the blocks are managed in these pools.

More Details
Apr 22, 2024
Using PGA_AGGREGATE_TARGET to Control Memory Allocation- Memory Structures

Earlier, I wrote that “in theory” we can use the PGA_AGGREGATE_TARGET to control the overall amount of PGA memory used by the instance. We saw in the last example that this is not a hard limit, however. The instance will attempt to stay within the bounds of the PGA_AGGREGATE_TARGET, but if it can’t, it won’t stop processing; rather, it will just be forced to exceed that threshold.

Another reason this limit is a “theory” is because work areas, though large contributors to PGA memory, are not the only contributors to PGA memory. Many factors contribute to PGA memory allocation, and only the work areas are under the control of the database instance. If you create and execute a PL/SQL block of code that fills in a large array with data in dedicated server mode where the UGA is in the PGA, Oracle can’t do anything but allow you to do it.

Consider the following quick example. We’ll create a package that can hold some persistent (global) data in the server:

$ sqlplus eoda/foo@PDB1

SQL> create or replace package demo_pkg as type array is table of char(2000) index b binary_integer; g_data array; end;/

Package created.

Now we’ll measure the amount of memory our session is currently using in the PGA/UGA (I used a dedicated server in this example, so the UGA is a subset of the PGA memory):

SQL> select a.name, to_char(b.value, ‘999,999,999’) bytes,

Initially, we are using about 6MB of PGA memory in our session (as a result of compiling a PL/SQL package, running this query, etc.). Now, we’ll run our query against T again using the same 300MB PGA_AGGREGATE_TARGET (this was done in a recently restarted and otherwise idle instance; we are the only session requiring memory right now):

SQL> set autotrace traceonly statistics; SQL> select * from t order by 1,2,3,4; 72616 rows selected.

As you can see, the sort was done entirely in memory, and in fact if we peek at our session’s PGA/UGA usage, we can see how much we used:

SQL> select a.name, to_char(b.value, ‘999,999,999’) bytes,

We see about 17MB of RAM being used. Now we’ll fill up that CHAR array we have in the package (a CHAR datatype is blank-padded so each of these array elements is exactly 2000 characters in length):

Now, that is memory allocated in the PGA that the instance itself can’t control. We already exceeded the PGA_AGGREGATE_TARGET set for the entire instance in this single session—and there is quite simply nothing the database can do about it. It would have to fail our request if it did anything, and it will do that only when the OS reports back that there is no more memory to give (ORA-04030). If we wanted, we could allocate more space in that array and place more data in it, and the instance would just have to do it for us.

However, the instance is aware of what we have done. It does not ignore the memory it can’t control; it simply recognizes that the memory is being used and backs off the size of memory allocated for work areas accordingly. So if we rerun the same sort query, we see that this time we sorted to disk—the instance did not give us the 12MB or so of RAM needed to do this in memory since we had already exceeded the PGA_AGGREGATE_ TARGET:

SQL> set autotrace traceonly statistics; SQL> select * from t order by 1,2,3,4;67180 rows selected.

So, because some PGA memory is outside of Oracle’s control, it is easy to exceed the PGA_AGGREGATE_TARGET simply by allocating lots of really large data structures in our PL/ SQL code. I am not recommending you do this by any means. I’m just pointing out that the PGA_AGGREGATE_TARGET is more of a request than a hard limit.

More Details
Nov 22, 2023
Flashback Database- Files

The FLASHBACK DATABASE command was introduced to speed up the otherwise slow process of point-in-time database recovery. It can be used in place of a full database restore and a rolling forward using archive logs, and it is primarily designed to speed up the recovery from an “accident.”

For example, let’s take a look at what a DBA might do to recover from an accidentally dropped schema, in which the right schema was dropped, just in the wrong database (it was meant to be dropped in the test environment). The DBA immediately recognizes the mistake they have made and shuts down the database right away. Now what?

Prior to the FLASHBACK DATABASE capability, what would probably happen is this:

\ 1.\ The DBA would shut down the database.

\ 2.\ The DBA would restore the last full backup of the database from tape (typically), generally a long process. Typically, this would be initiated with RMAN via RESTORE DATABASE UNTIL .

\ 3.\ The DBA would restore all archive redo logs generated since the backup that were not available on the system.

\ 4.\ Using the archive redo logs (and possibly information in the online redo logs), the DBA would roll the database forward and stop rolling forward at a point in time just before the erroneous DROP USER command. Steps 3 and 4 in this list would typically be initiated with RMAN via RECOVER DATABASE UNTIL .

\ 5.\ The database would be opened with the RESETLOGS option.

This was a nontrivial process with many steps and would generally consume a large piece of time (time when no one could access the database, of course). The causes of a point-in-time recovery like this are many: an upgrade script gone awry, an upgrade gone bad, an inadvertent command issued by someone with the privilege to issue it (a mistake, probably the most frequent cause), or some process introducing data integrity issues into a large database (again, an accident; maybe it was run twice instead of just once, or maybe it had a bug). Whatever the reason, the net effect was a large period of downtime.

The steps to recover, assuming you configured the Flashback Database capability, would be as follows:

\ 1.\ The DBA shuts down the database.

\ 2.\ The DBA startup-mounts the database and issues the Flashback Database command, using either an SCN (the Oracle internal clock), a restore point (which is a pointer to an SCN), or a timestamp (wall clock time), which would be accurate to within a couple of seconds.

\ 3.\ The DBA opens the database with resetlogs.

When flashing back, you can flashback either the entire container database (including all pluggable databases) or just a particular pluggable database. When flashing back the pluggable database, you would use restore points that you created for the entire database or for a specific pluggable database. Flashing back a pluggable database does not require that you open the database with resetlogs, but it does require that you issue the RECOVER DATABASE command while connected to the pluggable database you flashed back.

To use the flashback feature, your database must be in archivelog mode, and you must have a Fast Recovery Area (FRA) setup (because the flashback logs are stored in the FRA). To use normal restore points, you must enable flashback logging in the database. Guaranteed restore points do not require flashback logging in the database.

To view the flashback logging status, run the following query:

SQL> select flashback_on from v$database;
FLASHBACK_ON

NO

To enable flashback logging, do as follows in the root container:

$ sqlplus / as sysdba
SQL> alter database flashback on;

The final point here is that you need to set up the flashback capability before you ever need to use it. It is not something you can enable after the damage is done; you must make a conscious decision to use it, whether you have it on continuously or whether you use it to set restore points.

More Details
Oct 22, 2023
Fast Recovery Area (FRA)- Files

The FRA in Oracle is a location where the database will manage many of the files related to database backup and recovery. In this area (an area being a part of a disk set aside for this purpose; a directory, for example), you could find the following:

•\ RMAN backup pieces (full and/or incremental backups)
•\ RMAN image copies (byte-for-byte copies of datafiles and controlfiles)
•\ Online redo logs
•\ Archived redo logs
•\ Multiplexed control files
•\ Flashback logs

Oracle uses this new area to manage these files, so the server will know what is on disk and what is not on disk (and perhaps on tape elsewhere). Using this information, the database can perform operations like a disk-to-disk restore of a damaged datafile or the flashing back (a “rewind” operation) of the database to undo an operation that should not have taken place. For example, you could use the FLASHBACK DATABASE command to put the database back the way it was five minutes ago (without doing a full restore of the database and a point-in-time recovery). That would allow you to “undrop” that accidentally dropped user account.

To set up an FRA, you need to set two parameters: db_recovery_file_dest_size and db_recovery_file_dest. You can view the current settings of these parameters via

SQL> show parameter db_recovery_file_dest

NAME TYPE VALUE

db_recovery_file_dest string /opt/oracle/fra
db_recovery_file_dest_size big integer 100G

The Fast Recovery Area is more of a logical concept. It is a holding area for the file types discussed in this chapter. Its use is optional—you don’t need to use it, but if you want to use some advanced features, such as the Flashback Database, you must use this area to store the information.

Data Pump Files

Data Pump is a file format used by at least two tools in Oracle. External tables can load and unload data in the Data Pump format, and the import/export tools IMPDP and EXPDP use this file format.

They are cross-platform (portable) binary files that contain metadata (not stored in CREATE/ALTER statements, but rather in XML) and possibly data.

That they use XML as a metadata representation structure is actually relevant to you and me as end users of the tools. IMPDP and EXPDP have some sophisticated filtering and translation capabilities.

This is in part due to the use of XML and to the fact that a CREATE TABLE statement is not stored as a CREATE TABLE, but rather as a marked-up document.

This permits easy implementation of a request like “Please replace all references to tablespace FOO with tablespace BAR.” IMPDP just has to apply a simple XML transformation to accomplish the same. FOO, when it refers to a TABLESPACE, would be surrounded by FOO tags (or some other similar representation).

In Chapter 15, we’ll take a closer look at these tools. Before we get there, however, let’s see how we can use this Data Pump format to quickly extract some data from database A and move it to database B. We’ll be using an “external table in reverse” here.

External tables give us the ability to read flat files—plain old text files—as if they were database tables. We have the full power of SQL to process them. They are read-only and designed to get data from outside Oracle in. External tables can go the other way: they can be used to get data out of the database in the Data Pump format to facilitate moving the data to another machine or another platform. To start this exercise, we’ll need a DIRECTORY object, telling Oracle the location to unload to:

SQL> create or replace directory tmp as ‘/tmp’; Directory created.
SQL> create table all_objects_unload organization external
( type oracle_datapumpdefault directory TMPlocation( ‘allobjects.dat’ ))

asselect * from all_objects

Table created.

And that literally is all there is to it: we have a file in /tmp named allobjects.dat that contains the contents of the query select * from all_objects. We can peek at this information:

SQL> !strings /tmp/allobjects.dat | head x86_64/Linux 2.4.xxAL32UTF8 19.00.00.00.00001:001:000001:000001 i<?xml version=”1.0″ encoding=”UTF-8″?…

That’s just the head, or top, of the file. Now, using a binary SCP, you can move that file to any other platform where you have Oracle installed and by issuing a CREATE DIRECTORY statement (to tell the database where the file is) and a CREATE TABLE statement, such as this:

SQL> create table t ( OWNER VARCHAR2(30),OBJECT_NAME VARCHAR2(30),SUBOBJECT_NAME VARCHAR2(30),OBJECT_ID NUMBER, DATA_OBJECT_ID NUMBER,OBJECT_TYPE VARCHAR2(19),CREATED DATE,LAST_DDL_TIME DATE,TIMESTAMP VARCHAR2(19),STATUS VARCHAR2(7),TEMPORARY VARCHAR2(1),GENERATED VARCHAR2(1),SECONDARY VARCHAR2(1))

organization external

( type oracle_datapump default directory TMP location( ‘allobjects.dat’ ));

You would be set to read that unloaded data using SQL immediately. That is the power of the Data Pump file format: immediate transfer of data from system to system, over “sneakernet” if need be. Think about that the next time you’d like to take a subset of data home to work with over the weekend while testing.

Even if the database character sets differ (they did not in this example), Oracle has the ability now to recognize the differing character sets due to the Data Pump format and deal with them. Character set conversion can be performed on the fly as needed to make the data “correct” in each database’s representation.

Again, we’ll come back to the Data Pump file format in Chapter 15, but this section should give you an overall feel for what it is about and what might be contained in the file.

More Details
Sep 22, 2023
Flat Files- Files

Flat files have been around since the dawn of electronic data processing. We see them literally every day. The text alert log described previously is a flat file. I found the following definition for “flat file” on the Internet and feel it pretty much wraps things up:

An electronic record that is stripped of all specific application (program) formats. This allows the data elements to be migrated into other applica-tions for manipulation. This mode of stripping electronic data prevents data loss due to hardware and proprietary software obsolescence.

A flat file is simply a file whereby each “line” is a “record,” and each line has some text delimited, typically by a comma or pipe (vertical bar). Flat files are easily read by Oracle using either the legacy data loading tool SQLLDR or external tables. In fact, I will cover this in detail in Chapter 15.

Occasionally, a user will request data in a flat file format such as one of the following:

•\ Character (or comma) separated values (CSV): These are easily imported into tools such as spreadsheets.
•\ HyperText Markup Language (HTML): Used to display pages displayed in web browsers.
•\ JavaScript Object Notation (JSON): Standard text-based format for representing structured data and used for transmitting data in web browsers.

I’ll briefly demonstrate generating each of the file types in the following sections.

Generating a CSV File

You can easily generate CSV flat files from SQLPlus. You can either use the -m ‘csv on’ switch from the operating system prompt or use SET MARKUP CSV after starting a SQLPlus session, for example:

$ sqlplus scott/tiger@PDB1
SQL> set markup csv on delimiter , quote off SQL> SELECT * FROM dept;DEPTNO,DNAME,LOC

10,ACCOUNTING,NEW YORK

20,RESEARCH,DALLAS

30,SALES,CHICAGO

40,OPERATIONS,BOSTON

Here is an example of using the -m ‘csv on’ switch at the command line:

$ sqlplus -markup ‘csv on quote off’ scott/tiger@PDB1
SQL> select * from emp where rownum < 3;

EMPNO,ENAME,JOB,MGR,HIREDATE,SAL,COMM,DEPTNO 7369,SMITH,CLERK,7902,17-DEC-80,800,,20 7499,ALLEN,SALESMAN,7698,20-FEB-81,1600,300,30

Tip When you use the -markup ‘csv on’ switch, SQL*Plus sets variables such as ROWPREFETCH, STATEMENTCACHE, and PAGESIZE to optimize I/O performance.

Generating HTML

Similar to generating a CSV file, you can generate an HTML file from SQL*Plus by setting

SET MARKUP HTML ON before running your query:

SQL> set markup html on
SQL> select * from dept;

DEPTNODNAME… You can also specify the -markup ‘html on’ switch at the command line to generate HTML output: $ sqlplus -markup ‘html on’ scott/tiger@PDB1 In this manner, you can easily produce HTML output based on the contents of a table.
More Details
Aug 22, 2023
The Process Global Area and User Global Area- Memory Structures

The PGA is a process-specific piece of memory. In other words, it is memory specific to a single operating system process or thread.

This memory is not accessible by any other process or thread in the system. It is typically allocated via either of the C runtime calls malloc() or memmap(), and it may grow (or even shrink) at runtime.

The PGA is never allocated in Oracle’s SGA; it is always allocated locally by the process or thread—the P in PGA stands for process or program; it is not shared.

The UGA is, in effect, your session’s state. It is memory that your session must always be able to get to. The location of the UGA is dependent on how you connect to Oracle.

If you connect via a shared server, the UGA must be stored in a memory structure that every shared server process has access to—and that’s the SGA. In this way, your session can use any one of the shared servers, since any of them can read and write your session’s data.

On the other hand, if you are using a dedicated server connection, there’s no need for universal access to your session state, and the UGA becomes virtually synonymous with the PGA; it will, in fact, be contained in the PGA of your dedicated server.

When you look at the system statistics, you’ll find the UGA reported in the PGA in dedicated server mode (the PGA will be greater than or equal to the UGA memory used; the PGA memory size will include the UGA size as well).

So, the PGA contains process memory and may include the UGA. The other areas of PGA memory are generally used for in-memory sorting, bitmap merging, and hashing. It would be safe to say that, besides the UGA memory, these are the largest contributors by far to the PGA.

There are two ways to manage memory in the PGA: manual and automatic. The manual method should not be used (unless you’re on an old version of Oracle and don’t have a choice).

The automatic PGA memory management is the recommended technique that you should use. The automatic method is much simpler and more efficient in managing memory. The manner in which memory is allocated and used differs greatly in each case, so we’ll discuss each in turn.

More Details
Jul 22, 2023
Manual PGA Memory Management- Memory Structures

There are only a few scenarios that I can think of where manual PGA memory management may be appropriate. One is if you’re running an old version of Oracle and do not have the option to upgrade.

Another situation may be that you run large batch jobs that run during periods when they are the only activities in the instance where manual PGA management may provide performance benefits.

Therefore, it’s possible you may find yourself in an environment where this type of PGA memory management is used. Other than those scenarios, the manual management style should be avoided.

In manual PGA memory management, the following are the parameters that have the largest impact on the size of your PGA, outside of the memory allocated by your session for PL/SQL tables and other variables:

•\ SORT_AREA_SIZE: The total amount of RAM that will be used to sort information before swapping out to disk (using disk space in the temporary tablespace the user is assigned to).

•\ SORT_AREA_RETAINED_SIZE: The amount of memory that will be used to hold sorted data after the sort is complete. That is, if SORT_AREA_ SIZE is 512KB and SORT_AREA_RETAINED_SIZE is 256KB, your server process would use up to 512KB of memory to sort data during the initial processing of the query. When the sort is complete, the sorting area would “shrink” down to 256KB, and any sorted data that does not fit in that 256KB would be written out to the temporary tablespace.

•\ HASH_AREA_SIZE: The amount of memory your server process can use to store hash tables in memory. These structures are used during a hash join, typically when joining a large set with another set. The smaller of the two sets would be hashed into memory, and anything that didn’t fit in the hash area region of memory would be stored in the temporary tablespace by the join key.

Note Before using the parameters in the prior list, you must set the WORKAREA_ SIZE_POLICY parameter to MANUAL.

These parameters control the amount of space Oracle will use to sort or hash data in memory before using the temporary tablespace on disk, and how much of that memory segment will be retained after the sort is done. The SORT_AREA_SIZE (minus SORT_AREA_ RETAINED_SIZE) calculated value is generally allocated out of your PGA, and the SORT_ AREA_RETAINED_SIZE value will be in your UGA.

Here are the important things to remember about using the *_AREA_SIZE parameters:

•\ These parameters control the maximum amount of memory used by a SORT, HASH, or BITMAP MERGE operation.

•\ A single query may have many operations taking place that use this memory, and multiple sort/hash areas could be created. Remember that you may have many cursors opened simultaneously, each with its own SORT_AREA_RETAINED needs. So, if you set the sort area size to 10MB, you could use 10, 100, 1000, or more megabytes of RAM in your session. These settings are not session limits; rather, they are limits on a single operation, and your session could have many sorts in a single query or many queries open that require a sort.

•\ The memory for these areas is allocated on an “as needed” basis. If you set the sort area size to 1GB as we did, it doesn’t mean you’ll allocate 1GB of RAM. It only means that you’ve given the Oracle process the permission to allocate that much memory for a sort/hash operation.

Now that we’ve briefly reviewed manual PGA memory management, let’s move on to what you should be using, automatic PGA memory management.

More Details
Jun 22, 2023
Tablespaces- Files

As noted earlier, a tablespace is a container—it holds segments. Each segment belongs to exactly one tablespace. A tablespace may have many segments within it. All of the extents for a given segment will be found in the tablespace associated with that segment. Segments never cross tablespace boundaries.

A tablespace itself has one or more datafiles associated with it. An extent for any given segment in a tablespace will be contained entirely within one datafile. However, a segment may have extents from many different datafiles.

Graphically, a tablespace might look like Figure 3-3.

Figure 3-3.  A tablespace containing two datafiles, three segments, and four extents

Figure 3-3 shows a tablespace named USER_DATA. It consists of two datafiles, user_data01.dbf and user_data02.dbf. It has three segments allocated to it: T1, T2, and I1 (probably two tables and an index).

The tablespace has four extents allocated in it, and each extent is depicted as a logically contiguous set of database blocks. Segment T1 consists of two extents, one extent in each file. Segments T2 and I1 each have one extent depicted. If we need more space in this tablespace, we could either resize the datafiles already allocated to the tablespace or we could add a third datafile to it.

A tablespace is a logical storage container in Oracle. As developers, we will create segments in tablespaces. We will never get down to the raw file level—we don’t specify that we want our extents to be allocated in a specific file (we can, but in general we don’t). Rather, we create objects in tablespaces and Oracle takes care of the rest. If at some point in the future, the DBA decides to move our datafiles around on disk to more evenly distribute I/O, that is OK with us. It will not affect our processing at all.

Storage Hierarchy Summary

In summary, the hierarchy of storage in Oracle is as follows:

\ 1.\ A database is made up of one or more tablespaces.

\ 2.\ A tablespace is made up of one or more datafiles. These files might be cooked files in a file system, raw partitions, ASM-managed database files, or a file on a clustered file system. A tablespace contains segments.

\ 3.\ A segment (TABLE, INDEX, and so on) is made up of one or more extents. A segment exists in a tablespace, but may have data in many datafiles within that tablespace.

\ 4.\ An extent is a logically contiguous set of blocks on disk. An extent is in a single tablespace and, furthermore, is always in a single file within that tablespace.

\ 5.\ A block is the smallest unit of allocation in the database. A block is the smallest unit of I/O used by a database on datafiles.

More Details
Apr 22, 2023
Trace File Wrap-Up- Files

You now know the two types of general trace files, where they are located, and how to find them. Hopefully, you’ll use trace files mostly for tuning and increasing the performance of your application, rather than for filing service requests.

As a last note, Oracle Support does have access to many undocumented “events” that are very useful for dumping out tons of diagnostic information whenever the database hits any error. For example, if you are getting an ORA-01555 Snapshot Too Old that you absolutely feel you should not be getting, Oracle Support can guide you through the process of setting such diagnostic events to help you track down precisely why that error is getting raised, by creating a trace file every time that error is encountered.

Alert Log File

The alert file (also known as the alert log) is the diary of the database. It is a simple text file written to from the day the database is “born” (created) to the end of time (when you erase it). In this file, you’ll find a chronological history of your database—the log switches; the internal errors that might be raised; when tablespaces were created, taken offline, put back online; and so on. It is an incredibly useful file for viewing the history of a database. I like to let mine grow fairly large before “rolling” (archiving) it.

The more information, the better, I believe, for this file. I will not describe everything that goes into an alert log—that’s a fairly broad topic. I encourage you to take a look at yours, however, and see the wealth of information it holds.

To determine the location of the text-based alert log for your database, run the following query:

SQL> select value from v$diag_info, v$instance where name = ‘Diag Trace’;

VALUE

/opt/oracle/diag/rdbms/cdb/CDB/trace

The name of the alert log will be of this format:

alert_.log

You can generate the location and name of the alert log with the following query:

SQL> select value || ‘/alert_’ || instance_name || ‘.log’ from v$diag_info, v$instance where name = ‘Diag Trace’;

VALUE||’/ALERT_’||INSTANCE_NAME||’.LOG’

/opt/oracle/diag/rdbms/cdb/CDB/trace/alert_CDB.log
If you want to view the location of the XML-based alert log, run this query:
SQL> select value from v$diag_info where name = ‘Diag Alert’;

It’s worth noting that there is an internal table, X$DBGALERTEXT, that you can query from SQL*Plus which derives its information from the alert log. This table requires SYS privileges to view. For example, as SYS, you can query the alert log for ORA- errors as follows:

$ sqlplus / as sysdba
SQL> select record_id,
to_char(originating_timestamp,’DD.MM.YYYY HH24:MI:SS’),
message_text
from x$dbgalertext
where message_text like ‘%ORA-%’;
Tip Oracle Support has a note on how to edit, read, and query the alert log (Doc ID 1072547.1).

In addition to using an external table to query the alert log, you can easily view the alert log using the ADRCI tool. That tool lets you find, edit (review), and monitor (interactively display new records as they appear in the alert log). Also, Enterprise Manager provides access to the alert log.

Generally speaking, when the need arises to view the alert log, most DBAs will navigate directly to the alert log location. Once there, they’ll use operating system utilities such as vi, tail, and grep to extract information.

Tip With Oracle 19c and above, there’s also an attention.log file that contains information regarding critical events in your database (startup, shutdown, invalid memory parameters, and so on). It is located in the $ORACLE_BASE/diag/ rdbms///log directory.

More Details
Jun 22, 2022
The Storage Hierarchy in an Oracle Database- Files-2

The relationship between segments, extents, and blocks is shown in Figure 3-1.

Figure 3-1.  Segments, extents, and blocks

A segment is made up of one or more extents, and an extent is a logically contiguous allocation of blocks. A database may have up to six different block sizes in it.

Note  This feature of multiple block sizes was introduced for the purpose of making transportable tablespaces usable in more cases. The ability to transport a tablespace allows a DBA to move or copy the already formatted datafiles from one database and attach them to another—for example, to immediately copy all of the tables and indexes from an Online Transaction Processing (OLTP) database to a data warehouse (DW). However, in many cases, the OLTP database might be using a small block size, such as 2KB or 4KB, whereas the DW would be using a much larger one (8KB or 16KB). Without support for multiple block sizes in a single database, you wouldn’t be able to transport this information. Tablespaces with multiple block sizes should be used to facilitate transporting tablespaces; they are not generally used for anything else.

There will be the database default block size, which is the size specified in the initialization file during the CREATE DATABASE command. The SYSTEM tablespace will have this default block size always, but you can then create other tablespaces with nondefault block sizes of 2KB, 4KB, 8KB, 16KB, and, depending on the operating system, 32KB. The total number of block sizes is six if and only if you specified a nonstandard block size (not a power of two) during database creation. Hence, for all practical purposes, a database will have at most five block sizes: the default size and then four other nondefault sizes.

Any given tablespace will have a consistent block size, meaning that every block in that tablespace will be the same size. A multisegment object, such as a table with a LOB column, may have each segment in a tablespace with a different block size, but any given segment (which is contained in a tablespace) will consist of blocks of exactly the same size.

Most blocks, regardless of their size, have the same general format, which looks something like Figure 3-2.

Figure 3-2.  The structure of a block

Exceptions to this format include LOB segment blocks and hybrid columnar compressed blocks in Exadata storage, for example, but the vast majority of blocks in your database will resemble the format in Figure 3-2. The block header contains information about the type of block (table block, index block, and so on), transaction information when relevant (only blocks that are transaction managed have this information—a temporary sort block would not, for example) regarding active and past transactions on the block, and the address (location) of the block on the disk.

The next two block components are found on the most common types of database blocks, those of heap-organized tables. We’ll cover database table types in much more detail in Chapter 10, but suffice it to say that most tables are of this type.

The table directory, if present, contains information about the tables that store rows in this block (data from more than one table may be stored on the same block). The row directory contains information describing the rows that are to be found on the block.

This is an array of pointers to where the rows are to be found in the data portion of the block. These three pieces of the block are collectively known as the block overhead, which is space used on the block that is not available for your data, but rather is used by Oracle to manage the block itself.

The remaining two pieces of the block are straightforward: there may be free space on a block, and then there will generally be used space that is currently storing data.

Now that you have a cursory understanding of segments, which consist of extents, which consist of blocks, let’s take a closer look at tablespaces and then at exactly how files fit into the big picture.

More Details